So recently I've noticed that when seeding torrents, Transmission will frequently pause them with the warning "Error: Please Verify Local Data! Piece #____ is corrupt."

I'm wondering how serious of an issue this is. A little concerned it might be a failing hard drive (I'm downloading to an external HDD), but I've checked it in Disk Utility and it shows no errors. I don't want to shell out for a new drive if I don't have to or if that's not the real cause.

Is there something I should be doing to more thoroughly check the drive for failure? Or a way of double checking the torrented files to make sure Transmission's assessment is correct? (What does Transmission mean by "verify"?) Could this be an issue with downloading corrupted data from peers rather than the data becoming corrupted after the fact?