We're not allowed to throw computer parts into the garbage here... they have to be taken to a recycling center.  And people don't tend to like you going to a recycling center to take things away :)

Besides, for a render farm you'd probably want consistant sets of hardware, i.e. identical motherboards and processors etc.  Otherwise you're likely to spend too much time troubleshooting drivers etc which would be different for each system... and that kind of rubbish.

There are shops etc on ebay selling bulk recycled drives etc, for instance I found someone selling 10 (coincidentally!) 10gb drives very cheap... maybe this is the way to do it.  Find someone selling bulk reclaimed parts.