Migrating a VirtualBox Windows installation

I have been using Linux as my primary OS since 1999ish, except for a brief period early in the history of 67 Bricks when I had an iMac. Whenever I have used Windows it has invariably been in some kind of virtualised form; this was necessary in the iMac days when I was developing .NET applications in Visual Studio, but these days I work solely on Scala / Play projects developed in IntelliJ in Linux. Nevertheless, I have found it convenient to have an installation of Windows available for the rare instances where it’s actually necessary (for example, to connect to someone’s VPN for which no Linux client is available).

My Windows version of choice is the venerable Windows 7. This is the last version of Windows which can be configured to look like “proper Windows” as I see it by disabling the horrible Aero abomination. I tried running the Windows 10 installer once out of morbid curiosity, it started talking to me, so I stopped running it. I am old and set in my ways, and I feel strongly that an OS does not need to talk to me.

So anyway, I had a VirtualBox installation of Windows 7 and, because I am an extraordinarily kind and generous soul, I had given it a 250G SATA SSD all to itself using VirtualBox’s raw hard disk support.

Skip forward a few years, and I decided I would increase the storage in my laptop by replacing this SSD with a 2TB SSD since such storage is pretty cheap these days. The problem was – what to do with the Windows installation? I didn’t fancy the faff of reinstalling it and the various applications, and in fact I wasn’t even sure this would be possible given that it’s no longer supported. In any case, I didn’t want to give Windows the entire disk this time, I wanted a large partition available to Linux.

It turns out that VirtualBox’s raw disk support will let you expose specific partitions to the guest rather than the whole disk. The problem is that with a full raw disk, the guest sees the boot sector (containing the partition table and probably the bootloader), whereas you presumably don’t want the guest to see the boot sector if you’re only exposing certain partitions. How does this work?

The answer is that when you create a raw disk image and specify specific partitions to be available, in addition to the sda.vmdk file, you also get a sda-pt.vmdk file containing a copy of the boot sector, and this is presented to the guest OS instead of the real thing. Here, then, are the steps I took to clone my Windows installation onto partitions of the new SSD, keeping a partition free for Linux use, and ensuring Windows still boots. Be warned that messing about with this stuff and making a mistake can result in possibly irrecoverable data loss!

Step 1 – list the partitions on the current drive

My drive presented as /dev/sda

$ fdisk -x /dev/sda
Disk /dev/sda: 232.89 GiB, 250059350016 bytes, 488397168 sectors
Disk model: Crucial_CT250MX2
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0xc3df4459

Device     Boot  Start       End   Sectors Id Type            Start-C/H/S   End-C/H/S Attrs
/dev/sda1  *      2048    206847    204800  7 HPFS/NTFS/exFAT     0/32/33   12/223/19    80
/dev/sda2       206848 488394751 488187904  7 HPFS/NTFS/exFAT   12/223/20 1023/254/63 

Step 2 – create partitions on the new drive

I put the new SSD into a USB case and plugged it in, whereupon it showed up as /dev/sdb

Then you need to do the following

  1. Use fdisk /dev/sdb to edit the partition table on the new drive
  2. Create two partitions with the same number of sectors (204800 and 488187904) as the drive you are hoping to replace, and then you might as well turn all of the remaining space into a new partition
  3. Set the types of the partitions correctly – the first two should be type 7 (HPFS/NTFS/exFAT), if you’re creating another partition it should be type 83 (Linux)
  4. Toggle the bootable flag on the first partition
  5. Use the “expert” mode of fdisk to set the disk identifier to match that of the disk you are cloning; in my case this was 0xc3df4459 – Googling suggested that Windows may check for this and some software licenses may be tied to it.

So now we have /dev/sdb looking like this:

$ fdisk -x /dev/sdb
Disk /dev/sdb: 1.82 TiB, 2000398934016 bytes, 3907029168 sectors
Disk model: 500SSD1         
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xc3df4459

Device     Boot     Start        End    Sectors Id Type            Start-C/H/S End-C/H/S Attrs
/dev/sdb1  *         2048     206847     204800  7 HPFS/NTFS/exFAT     0/32/33 12/223/19    80
/dev/sdb2          206848  488394751  488187904  7 HPFS/NTFS/exFAT   12/223/20 705/42/41 
/dev/sdb3       488394752 3907029167 3418634416 83 Linux             705/42/42 513/80/63

Note that the first two partitions are an exact match in terms of starts, end and sector count. I think we can ignore the C/H/S (cylinder / head / sector) values.

Step 3 – clone the data

Taking great care to get this right, we can use the dd command and we can tell it to report its status from time to time so we know something is happening. There are two partitions to clone:

$ dd if=/dev/sda1 of=/dev/sdb1 bs=1G status=progress
... some output (this is quite quick as it's only 100M)
$ dd if=/dev/sda2 of=/dev/sdb2 bs=1G status=progress
... this took a couple of hours to copy the partition, the bottleneck probably being the USB interface

Step 4 – create (temporarily) a VirtualBox image representing the new disk

There are various orders in which one could do the remaining steps, but I did it like this, while I still had /dev/sda as the old disk and the new one plugged in via USB as /dev/sdb

$ VBoxManage internalcommands createrawvmdk -filename sdb.vmdk -rawdisk /dev/sdb -partitions 1,2

This gives two files; sdb.vmdk and sdb-pt.vmdk. To be honest, I got to this point not knowing how the boot sector on the disk would show up, but having done it I was able to verify that sdb-pt.vmdk appeared to be a copy of the first sector of the real physical disk (at least the first 512 bytes of it). I did this by comparing the output of xxd sdb-pt.vmdk | head -n 32 with xxd /dev/sdb | head -n 32 which uses xxd to show a hex dump and picks the first 32 lines which happen to correspond to the first 512 bytes. In particular, you can see at offset 0x1be the start of the partition table, which is 4 blocks of 16 bytes each ending at 0x1fd, with the magic signature 55aa at 0x1fe. The disk identifier can also be seen as the 4 bytes starting at 0x1b8. And note that everything leading up to the disk identifier is all zeroes.

Step 5 – dump the boot sector of the existing disk

At this point, we have /dev/sda with the bootsector containing the Windows bootloader, we have a freshly initialised /dev/sdb containing a copy of the Windows partitions and a bootsector that’s empty apart from the partition table and disk identifier, and we have the sdb-pt.vmdk file containing a copy of that empty bootsector. We have also arranged for the new disk to have the same identifier as the old disk.

What is needed now is to create a new sdb-pt.vmdk file containing the bootloader code from /dev/sda and then the partition table from /dev/sdb. There are 444 bytes that we need from the former. So we can do something like this:

$ head -c 444 /dev/sda > sdb-pt-new.vmdk
$ tail -c +445 sdb-pt.vmdk >> sdb-pt-new.vmdk

We can confirm that the new file has the same size as the original, we can also use xxd to confirm that we’ve got the bootloader code and then everything from that point is as it was (including the new partition table)

Step 6 – the switch

We’re almost done. All that’s left to do is:

  1. Replace the old SATA SSD with the new 2TB SSD
  2. Run VirtualBox – do not start Windows but instead remove the existing hard drive attached to it (which is the “full raw disk” version of sda.vmdk), and use the media manager to delete this.
  3. Quit VirtualBox
  4. Recreate the raw disk with partitions file but for /dev/sda (which is what the new drive is): VBoxManage internalcommands createrawvmdk -filename sda.vmdk -rawdisk /dev/sda -partitions 1,2
  5. The previous command will have created the sda-pt.vmdk boot sector image from the drive, which will again be full of zeroes. Overwrite this with the sdb-pt-new.vmdk that we made earlier, obviously ensuring we preserve the original sda-pt.vmdk name and the file permissions.
  6. Start VirtualBox, add the newly created sda.vmdk image to the media manager, and then as the HD for the Windows VM
  7. Start the Windows VM and hope for the best

I was very gratified to find that this worked more-or-less first time. In truth, I originally just tried replacing the old sda.vmdk with the new one, which gave the error “{5c6ebcb7-736f-4888-8c44-3bbc4e757ba7} of the medium ‘/var/daniel/virtualbox/Windows 7/sda.vmdk’ does not match the value {e6085c97-6a18-4789-b862-8cebcd8abbf7} stored in the media registry (‘/home/daniel/.config/VirtualBox/VirtualBox.xml’)”. It didn’t seem to want to let me remove this missing drive from the machine using the GUI, so I edited the Windows 7.vbox file to remove it, then removed it from the media library and added the replacement, which I was then able to add to the VM.

Programming a Tesla

I have a friend called Chris who is a big fan of the band China Drum. Many years ago he challenged me to program their song Last Chance as a custom ringtone on his Nokia phone and, being vaguely musical, I obliged.

Time has moved on since then. With his hitherto rock-star hair cut to a respectable length, he is now the CEO of a company providing disease model human cells. And he owns a Tesla, something he likes to remind me about from time to time. Now it turns out that one of the silly things you can do with a Tesla is program the lights to flash to make a custom light show for a piece of music of your choice. You can probably see where this is heading.

Since a) I was going up to meet him (and his Tesla) in Yorkshire last weekend and b) I wasn’t sure how long the “I’ve been busy” excuse would work in response to the “Where’s my light show?” question, I figured I’d probably better actually try to do the damn thing.

Problem 1 – I don’t have the song

Sadly I didn’t have a copy of China Drum’s Goosefair album, so I didn’t have the audio. But I do have Linux, and a Spotify subscription, and the command-line ncspot client. I reasoned that if I could play the audio, I could surely also record the audio, it just might mean losing the will to live while trying to understand how Pulseaudio works (or maybe it’s PipeWire now, or who knows?)

Cutting a long and tedious story short, by doing some mystical fiddling about, I was able to send the output of ncspot to Audacity and thereby record myself a WAV of the song. In the interests of remaining on the right side of the law, I made good faith attempts to locate an original copy of the album, but it seems to be out of print. So I now have a copy via eBay.

Problem 2 – I don’t have the application for building the light show

To build a light show, one needs an open-source application called xLights. Since this doesn’t appear to be in the Ubuntu package repository, I had to build it from source. For some reason, I’m a bit averse to installing random libraries and things on my machine, but fortunately there is a “build it in Docker” option which I used and seemed to work successfully, except that I couldn’t figure out how to get at the final built application! It existed as a file in the filesystem of the docker container, but since the executing script had finished, the container wasn’t running, and there seemed to be no obvious way to get at it (it is entirely possible that I was just being stupid, of course). In the end, I reasoned that any file on a container filesystem has to be in my /var/lib directory somewhere, and with a bit of poking around, I located the xLights-2023.08-x86_64.AppImage file and copied it somewhere sensible.

Problem 3 – I don’t know where the beats are

I followed various instructions and got myself set up with a working application, a fresh musical sequence project for the file, and the .wav imported and displaying as a waveform.

The way xLights works is that you start with a bunch of horizontal lines representing available lights, you set up a bunch of timing markers which present as vertical lines thus dividing the work area up into a grid, and you can create light events in various cells of the resulting grid and the start / stop transitions of each light are aligned with the timing markers (though they can subsequently be moved). Thus you need a way of creating these markers. Fortunately, it is possible to download an audio plugin to figure these out for you. After doing this, you end up with a screen looking something like this:

An empty xLights grid.

Problem 4 – I can’t copy and paste

So I started filling things in with the idea that if I got something that I was happy with for a couple of bars, I could copy and paste it elsewhere rather than having to enter every note manually. Unfortunately, I ran into an unexpected problem, which is that the timing on the track is very variable. For example, I picked a couple of channels and used one to show where the “1” beats were in each bar and put lights on “2”, “3” and “4” in the other. But if you then try to copy and paste this bar into two bars, then four, then eight and so on, you quickly get out of sync with the beat lines, because the band speed up as the song goes on. Also, I couldn’t see any obvious option in xLights to quantise a track (i.e. to adjust the starts and ends of notes to match a set of timing marks).

Fortunately, the format that xLights uses to save these sequences is XML-based. Therefore I was able to write a Scala application to read in the sequence, make a note of all of the timestamps corresponding to the timing marks, and then shuffle all starts and ends of notes to the nearest timestamp. Actually, originally I wrote a thing to try to regularise the timestamp markers (i.e. keep the same number of them but make the spacing uniform) and quickly realised that the resulting markers were woefully out of sync with the music, which is when I realised the tempo of the song was variable.

Problem 5 – I can’t count

I dimly remembered from the Tesla light-show instructions that there was a limit of 3500 lights in a show, so I had to make sure the total number didn’t go above that. I couldn’t see an obvious way to do this in xLights, but I could see that it was just a question of running a suitable XPath expression against the XML file. So I fired up oXygen and wrote one (the correct XPath – see below – is count(//Effect[not(parent::EffectLayer/parent::Element[@type='timing'])]))

And so my development cycle was basically:

  • Stare at lots of blobs on the grid, adding and removing them and generally twiddle until it seems satisfactory
  • Run the Scala application to quantise it all to the timing marks, thus dealing with any slight mismatches caused by copying-and-pasting
  • Check in oXygen how many lights I’ve used (editing the raw XML was also useful to copy between channels e.g. to make the rear left turn indicators do the same as the front left turns)

Problem 6 – I can’t read instructions

Having got something I was reasonably happy with, I double-checked the instructions. Oh no! I am an idiot! It doesn’t say 3500 lights, it says 3500 commands where a command is “turn light on” or “turn light off”. So I now have twice the number of allowed lights, and drastic editing would be needed (naturally, this was the Friday night before I was due to drive up with it, after a number of late nights working on it).

Fortunately, I had also been an idiot (very slightly) with my counting XPath; because of the way the XML format works, each timing mark was being counted as an event. So having tweaked it not to count the timing marks, I had around 2600 lights, which is rather more than the 1750 budget but less bad than the 3400ish I started with.

So I had to scale things back a bit. I dropped the tracks I’d been using for the “1 2 3 4” beat. I removed some of the doubling up between channels. I dropped some of the shorter notes in tracks where I’d been trying to reproduce the rhythm of the vocals. I simplified bits and generally chopped it about until I had something that met the requirements.

Part of the finished product

Problem 7 – I don’t have a USB stick

Well, I didn’t at the start of the project. I did by the end; the smallest one I could find in Curry’s was 32GB. Which is ridiculous overkill for the size of the files needed (the audio was 27M, the compiled lightshow file was 372K). But, well, whatever…

And that was it. I followed the instructions about what needed to be on the stick (a folder called “LightShow” with that exact capitalisation containing “lightshow.wav” and “lightshow.fseq”) and took it with me to Yorkshire. Chris plugged it into the Tesla. It ran successfully. Hooray!

I’m not sure there’s a great moral to this story (other than maybe “read the instructions carefully”) but it was a fun challenge. Thanks to China Drum for a great song, Tesla for building the light show feature, and the xLights authors for an open-source application that made building the light show possible.

Embracing Impermanence (or how to check my sbt build works)

Stable trading relationships with nearby countries. Basic human rights. A planet capable of sustaining life. What do these three things have in common?

The answer is that they are all impermanent. One moment we have them, the next moment – whoosh! – they’re gone.

Today I decided I would embrace our new age of impermanence insofar as it pertains to my home directory. Specifically, I wondered whether I could configure a Linux installation so that my home directory was mounted in a ramdisk, created afresh each time I rebooted the server.

Why on earth would I want to do something like that?

The answer is that I have a Scala project, built using sbt (the Scala Build Tool), and I thought I’d clear some of the accumulated cruft out of the build.sbt file, starting with the configured resolvers. These are basically the repositories which will be searched for the project’s dependencies – there were a few special case ones (e.g. one for JGit, another for MarkLogic) and I strongly suspected that the dependencies in question would now be found in the standard Maven repository. So they could probably be removed, but how to check, since all of the dependencies would now exist in caches on my local machine?

A simple solution would have been to delete the caches, but that involves putting some effort into finding them, plus I have developed a paranoid streak about triggering unnecessary file writes on my SSD. So I had a cunning plan – build a VirtualBox VM and arrange for the home directory on it to be a ramdisk, thus I could check the code out to it and verify that the code will build from such a checkout, and this would then be a useful resource for conducting similar experiments in the future.

Obviously this is not quite a trivial undertaking, because I need some bits of the home directory (specifically the .ssh directory) to persist so I can create the SSH keys needed to authenticate with GitHub (and our internal GitLab). Recreating those each time the machine booted would be a pain.

After a bit of fiddling, my home-grown solution went something like this:

  • Create a Virtualbox VM, give it 8G memory and a 4G disk (maybe a bit low if you think you’ll want docker images on it; I subsequently ended up creating a bigger disk and mounting it on /var/lib/docker)
  • Log into VM (my user is daniel), install useful things like Git, curl, zip, unzip etc.
  • Create SSH keys, upload to GitHub / GitLab / wherever
  • Install SDKMAN! to manage Java versions
  • Create /var/daniel and copy into it all of the directories and files in my home directory which I wanted to be persisted; these were basically .ssh for SSH keys, .sdkman for java installations, .bashrc which now contains the SDKMAN! init code, and .profile
  • Save the following script as /usr/local/bin/create_home_dir.sh – this wipes out /home/daniel, recreates it and mounts it as tmpfs (i.e. a ramdisk) and then symlinks into it the stuff I want to persist (everything in /var/daniel)

mount | grep $DIR && umount $DIR

[ -d $DIR ] && rm -rf $DIR

mkdir $DIR

mount -t tmpfs tmpfs $DIR

chown -R daniel:daniel $DIR

ls -A /var/daniel | while read FILE
  sudo -u daniel ln -s /var/daniel/$FILE $DIR/
  • Save the following as /etc/systemd/system/create-home-dir.service
description=Create home directory


  • Enable the service with systemctl enable create-home-dir
  • Reboot and hope

And it turns out that this worked; when the server came back I could ssh into it (i.e. the authorized_keys file was recognised in the symlinked .ssh directory) and I had a nice empty workspace; I could git clone the repo I wanted, then build it and watch all of the dependencies get downloaded successfully. I note with interest that having done this, .cache is 272M and .sbt is 142M in size. That seems to be quite a lot of downloading! But at least it’s all in memory, and will vanish when the VM is switched off…