however I can still seed the torrent how is that possible?
Yes you can still seed as well as download. But you are limited and can only upload and download torrent data in swarms that contain peers that are themselves fully connectable (port forwarded).
So say you join a torrent swarm that only contains peers just like you (firewalled, no ports forwarded) then no one will transfer any torrent data with each other. Everyone is stuck waiting for a fully connectable (port forwarded) peer to join that swarm.
Should be fine, just don't cheap out on the external drive / cable you will be using. And when you're using something like smartctl you'll know right away if SMART info is passing through your USB for proper testing.
I've done a lot of these type of scans via USB drives, honestly the more annoying part is that some USB drives do wonky things like go into sleep mode within 1-5 minutes which will disrupt any sort of scanning you had going. So with USB drive scanning I usually implement something to keep the drive alive and awake e.g. a simple infinite loop script to write a file every x seconds, or if you're on windows you can also use KeepAliveHD.