_AsyncCopying folder

i think i understand what that folder does, keeps the data safe in case of anything going wrong when moving events around from one library to another....but it is now 400gb and i need to move events to the external library to free up space but that adds size to the AsyncCopying folder.....why does fcpx not just get rid of these files once the transfer is complete and the files are in the new location? or maybe hold on to them for a little while? going in and deleting files does not seem to be a good idea since all this is part of the larger database.....any ideas?