1) Copy + delete (when moving a file from one filesystem to another) or

2) Update the relevant metadata when moving a file from one place on a filesystem to another place on the same filesystem

In simple terms, moving a file around on the same filesystem can be an atomic operation whereas moving a file from one filesystem to another is not (because it involves copying + deleting).

I can only think that you're running into an issue where the OS is protecting you from what would happen if, halfway through the copy+delete, someone else on the same system (it's multi-user, remember) tried to do something that might conflict with your actions that are in progress.

XP probably doesn't care if your data gets trashed en route if someone nips in and deletes the file that you are in the middle of moving to another filesystem, so that the next time your process goes to the disk for the next few MB to buffer up, there's nothing there with that filename any more that it can read from.

Pure speculation on my part, of course.
--
simon