Undo copy can't redo...FML

Now tell me why it's an option to undo a copy of a file when the medias original location area is missing? So I had an export of an edit that i brought into final cut from last night.... this morning I wanted to undo an edit in fcp, I didn't come back from finder(I move kinda fast) and hit command+z. It undid the copy of the Media to my scratch drive, hence removing it from the project, but the thumb drive which the media was on was cleared..... now it can redo the copy because there's nothing to copy from. *? Why would it have given me the undo copy in the first place?