When you download a torrent (Ubuntu ISOs for example...), you usually want to contribute some upload back into the torrent (I think a 1:1 ratio is preferred, and 1:0.5 [DL:UL] some kind of street avg. minimum?)
So, I just happened to try to seed a torrent from my Trash folder. If you're like me, you like to download stuff to your Desktop to later process it as you see fit. Once I've moved my stuff around, I trash what's on my desktop. But now, I still want Transmission (my fav torrent client) to seed that.
