Upload & Sell: On
Around here, digital requires 3 copies of data to be considered "backed up", one of which is kept off site. For me, that means 3 copies total, most of which go on hard drives. I have one internal drive that I archive to and a number of externals. I also burn DVDs occasionally, depending on the project and the mood I'm in (yes, I'm a bit erratic.)
FWIW, I consider it a good practice to buy my archiving drives from different manufacturers, or at least from different lots if from the same manufacturer. A run of bad HDs is not all that common, but it's sure not unheard of either.
Also, just because a HD is full doesn't mean you don't need to spin it up once in a while. My hardware geek friend says that just leaving a full HD on the shelf for months / years can end up being a very bad idea.