I went with an array of 1TB drives, which works well for my needs. I have redundant backups of EVERYTHING, on separate hard drives, all online and accessible instantly at hard drive bus speeds via my eSATA interface, and if one of my hard drives goes bad, I know it right away and am able to replace it, copy the backup to it, and have my redundant assurance/insurance again quickly. This has happened already, and it was painless and easy to recover.
At the moment, I'm running with a 10TB hard drive system:
Here are the external drive enclosures:
Each of the Addonics enclosures holds 4 drives, and I've stuffed them with eight 1 Tb drives total. The top two Antec enclosures are single-drives, and they're each holding a 750 Gig drive. The final 500 Gigs is in my main system unit, but only used for operating system and programs. So, 10 TB at the moment, but a totally expandable system if I want to add more later.
Those 10 drives/cables all plug into a pair of e-SATA port multipliers (that's just an unrelated USB hub on top, for running my mouse and keyboard and importing data from my Photo Geo-tagging device):
Each port multiplier reduces 5 drives/cables down to 1:
And those two resulting cables are then plugged into a card on the motherboard of the main system unit.
To facilitate the back ups, I use a program called
Allway Sync and it works really well for me. I just open it up every time I dump new photos onto my hard drive or do some editing that I don't want to have to repeat, and hit "Synchronize".
At the end of each month I burn the newest photos to 2 DVDs (or sets of DVDs if there are too many to fit on one); One goes in the safe and one goes to my sister's house for safe keeping.
That's what I've been using for the past several years, and it's working pretty well for me so far.