I may be a bit biased as a friend work for the company, but have you had a look at the Drobo?
It's main feature is that it looks like a single large drive to the host, even though it might not have that physical disk capacity, so start off with a single 250GB drive, and it looks like a 2TB drive to the host. When the physical drive gets full (LEDs on the front tell you the 'real' capacity), then just slot in another drive. If it has enough drives, then it will automatically use mirroring/RAID over the drives too, so if you've filled it with 4 x 250GB drives, then you just pop one of the
250GB drives out and pop in a 500GB drive and it "just works", and so on.They're not as cheap as just a drive in a box though, however if you need the capacity and flexability, they might be good for you...
The Drobo is, and it spins down the disks when not in-use too.
Gordon