For long-winded reasons I'd rather not explain nor defend, my partner uses Dropbox for a large set of important files (~70G), which are stored on a removable drive. She is also a Windows user. Recently, by introducing a few new drives, I inadvertently changed the drive letter that is assigned to her portable device, which stopped Dropbox from working. Solving this properly is rather finicky so I thought I'd write down what I did.

The end goal is to try and ensure that the portable drive always gets the same drive letter and that Dropbox is configured to use that drive letter, but before I can get that far I need to get Dropbox syncing again. It used to be F:, and I opted for U: going forward.

Luckily the extra letters I'd introduced were all partitions on a separate hard drive from the OS, so I powered the machine down, unplugged the extra hard drive and booted back up. This freed up the stolen drive letters, but the portable drive did not re-inherit them. Running regedit as an administrative user and renaming keys in HKEY_LOCAL_MACHINE\SYSTEM\MountedDevices moved the USB drive back to the correct letter, but I suspected the extra hard drive would take precedence when it was back, so this was only an interim solution.

With the portable drive back on F:, the Dropbox client was happy once again. However I needed to reconfigure the Dropbox client to use a different letter. I wasn't happy with the idea of altering the Dropbox client's configuration database under its feet, so I had to do everything "by the book". Luckily, the client supports the notion of moving your Dropbox folder. Combined with the command-prompt subst command (run as the local user rather than as an admin this time), I was able to clone the F: drive to a virtual U: drive, and then ask Dropbox to move the folder.

subst U: F:\

This was pretty awkward. F:\Dropbox and U:\Dropbox are in fact the same folder, so I needed to ask Dropbox to move it from F: to something other than U:. I opted for U:\tmp. The move took a long time (~2 hours).

The proper solution is to try and get a stable drive letter for her device. This can be achieved using a tool called 'USB Drive letter Manager', or USBDLM. USBDLM is free for educational use, and my partner is a teacher, which is lucky.

Once that's all sorted, reboot, insert portable device, ensure it's on the right drive letter, and move the Dropbox folder back down to U:. This time the move was near instantaneous.