The problem with using hard drives for this purpose--apart from their individual failure rate and somewhat limited service life--is that hard disk interfaces change with the times. Does anyone remember ESDI? My first large hard drive in the early 1990s (a full-height 700 MB Maxtor, costing well over $2000 in today's dollars) used that technology. If that drive still existed, I don't think I could find any way to connect it up now, some 20 years later. My UltraStor (company long out of business) ESDI controller fit only a VLB slot--an expansion card standard for i486 computers, which was quickly abandoned when the Pentium processor was released.
I'm still using Windows XP with a PATA (EIDE) drive as my C: drive. I'm currently looking to build a new Core i5-based PC to run Windows 7, but I notice that a growing number of motherboards in that class don't have on-board support for PATA or floppy drives any more. Hmmm.
Some day the present-day SATA interface will be similarly obsolete. At that point, I suppose the thing to do would be copy all your old hard drives onto whatever seems headed for greatness at that point. But I have reasonable confidence in being able to play back a DVD-R ~20 years from now, at least the single-layer kind. I've had mixed results with dual-layer DVD-Rs.