I don't what client you are using, but this works with uTorrent. First, put the drive back in, or put all files back into the whatever the default download location is for your client. Then delete the .torrent file (NOT the files in the torrent). Then re-download the .torrent file. Your client should recognize you already have all the files in the proper location, so it will check them to make sure none are corrupt and then it will begin seeding.