[lgerard]

it depends

if the torrents are still active on the tracker, you need only download the torrent file from the original thread and point it to the audio or video files. it is posible to seed from an external drive, but I would not recomend it. it would be better to move or copy the files to your laptop, and move them back (or delete the copy) when you are done.
some external drives do not have the best ventilation or power and I would not let mine run for long periods of time.

if the torrents are off the tracker, or from another site you would have to create a new torrent and upload it to the tracker.