If you're truly interested in occassional access and backups are your focus, you might look at a network attached storage device like this: http://www.infrant.com
The bonus is that the device is RAID 5 so you have some data protection. I've had one for about 4 months and am slowly working it into my system.(I'm trying to complete my file organization before I migrate everything to the NAS) I only wish I'd started with 400GB drives as I currently have it filled with 250s.
Just to add--there are barebones models that you can install your HDs in. Cost is around $600.