If you're moving data from one Mac or PC to a Mac running Mac OS X Leopard, don't trip over the cord.
Tom Karpik, a computer science student at the British Columbia Institute of Technology, says he has identified a bug in Leopard that can cause you to lose data if problems occur while moving files between two different storage volumes, such as two different hard drives. Apparently, the problem is that if the source of the file crashes or is disconnected from the network while the move is under way, the contents of the file completely disappear from the source machine, and you're only left with whatever had already been copied to the destination machine.
We're talking about moving files here, not copying them, which is a bit more dangerous way of getting a file from one place to another. When you opt to move a file, you're acknowledging that you want that file deleted from the source machine after the operation is completed. And you have to actually hold down the Command button when dragging the file from one volume to another to bypass the default way of doing this operation, which is to "copy" the file from source to destination when the icon is dragged across the screen.
Commenters on Karpik's blog, Slashdot, and others like Daring Fireball's John Gruber point out that you're asking for trouble if you opt to "move" files rather than copy them. An easy way to avoid any problems is to just drag the file from one volume to another without pressing any keys, and if you really want the file gone from the source machine, manually delete it after everything has been completed.
But Karpik's point is that Leopard shouldn't delete the source data until it has verified that the transfer was successfully completed. "Windows behaves differently in that it never deletes the source unless everything has been completely copied to the destination. This is just sane, to-be-expected behaviour," he wrote in an e-mail in response to asking for clarification on a couple of his findings. Some commenters on Karpik's blog said this is an issue that has existed since Panther (Mac OS X 10.3), but I haven't been able to confirm that.
I sent an e-mail to Apple earlier this morning asking if it had identified this as a bug or problem, and if any fix was anticipated. I'll update if I hear back.