Worst. OS Bug. Evar.
So, you use Finder to pick a folder to move to a different volume and start the moving process. If however, something happens to cause that volume to go offline during the move process then your original copy of the data in the folder that you were moving is deleted as though the move process was completed successfully. Bye-bye data. Any files not moved to the destination volume are lost. I’m guessing that the OS assumes that the move operation has completed correctly and cleans up after itself, unwittingly deleting files that haven’t been moved.
This is the sort of bug that’ll make you appreciate that you took the time to become acquainted with Time Machine.
Um, no. It's the kind of bug that makes me appreciate not taking any time at all to become acquainted with Time Machine, Finder, or Leopard.