Inside each X:\$RECYCLE.BIN folder is a set of additional folders, one per user, that are the per-user Recycle Bins. Explorer hides their real names from you, but you can see them from the Command Prompt if you so desire. (They are just the SIDs of the users.) That's why you're seeing multiple Recycle Bins per drive. You can just delete these directly with no ill effects, and you can do that per-drive.