Mac OS X Finder users moving (as opposed to copying) files from one volume or drive to the other may be in for a rude surprise if the connection is interrupted during a file transfer. Tom Karpik outlines this bug and how to reproduce it on his blog. If you’re moving files or folders from one drive or mounted share to another, and if the connection--USB, network, etc...--is interrupted, the original folders or files will disappear from existence. It isn’t simply a matter of files disappearing from the Finder, you won’t be able to access them from the Terminal, either. They will be, for all intents and purposes, gone. But you back up regularly, right?

The worst part about this? According to those who have commented on Karpik’s article, this bug has been around since Panther (Mac OS X 10.3).

As I mentioned before, this only impacts moving items between volumes. By default, the Finder will copy items to the target disk, leaving the original intact. If you hold down the command key while dragging a file to another disk, the files will be moved outright instead. This is when this bug comes into play. Karpik gives a short primer on what happens behind the scenes when Finder copies and moves files, which is worth reading.

John Gruber noted a workaround: copy the file to the other disk and then deltete the original if the copy job is successful.

I tried using the mv command in Terminal while forcibly interrupting the connection to the target disk (yanking out the USB cable) and as far as I could tell, it didn’t result in any data loss. Can anybody verify this for me?

Here’s hoping Apple finally notices this bug and fixes it in 10.5.1.