Never, ever use a single hard drive for photo storage. You need at least two for redundancy.
Either poor man's RAID (copy the files to each drive), or real RAID. Poor Man's RAID is less convenient but lets you physically separate the drives, and is more immune to operating system/operator errors that might cause data loss.
Of course, the best is a combination of poor man's RAID using multiple RAID arrays.
Right now I'm using a relatively old RAID-5 array of 5x 250GB drives (giving 1TB usable storage since one drive is parity). I need to add a secondary backup mechanism to this, I have been lax in that regard.
I am considering Blu-Ray when the prices drop, but DVD-R in the meantime.