Hard drives are nice, but they do go south. It's not if, it's when. Though I too have lots of images stored on high capacity hard disks, (around a terabyte or so) most of my images are archived on high capacity tape which is used as near-line and backed by magneto-optical for on-line storage.
Both tape an magneto-optical have a considerably longer unrefreshed shelf life than hard disk's magnetic storage. High end tape (I use Ultrium LTO) has a storage life expectancy of around 35 years and magneto-optical more than doubles that without the potential deterioration of plastic media like DVD.
I have several terabytes of photos archived to Ultrium media. If you are not in the photography business, it's probably overkill and a bit too expensive (about 5K for a tape drive) but if your photos are valuable to not only you but your customers, you can easily justify the expense of redundancy and the best technology possible.
Some of the latest tape technology such as Ultrium LTO (linear tape open) and SDLT (Super Digital Linear Tape) have tremendous storage capacity per tape. The SDLT stores over 600 gigabytes per tape in compressed mode and has a transfer rate of 72 megabytes per second so that backups of 100 gigabyte hard disks takes about 15 minutes.