Wow. I'm getting quite an education here. Some of you are clearly more technically proficient than I am. Let's talk budget and an approach that would allow incremental capacity upgrades later. If I could only spend $600 or less and wanted the simplest solution, where would you recommend I spend the money?
On a Mac? Two RAID 1 arrays -- one as your working drive, one as a local Time Machine backup, and one more external as an offsite copy.
On a PC, same deal, just using a different backup application than Time Machine.
Assuming that you have less than 2TB of data, this shouldn't be a problem. Remember that because RAID 1 is mirrored storage you'll have to buy 4TB of physical storage to hit 2TB.
You could buy two of these --
http://www.bhphotovideo.com/c/product/625843-REG/Western_Digital_WDH2Q40000N_4TB_My_Book_Studio.html -- and use this (
http://www.bhphotovideo.com/c/produ...TAY2000102_Expansion_External_Hard_Drive.html) as your offsite copy.
When you run out of space on your working drive, buy a new RAID 1 that is double the capacity of the previous, turn that into your Time Machine/backup drive, and turn your old Time Machine/backup drive into a second volume for your Lightroom library. If you're using Aperture it might be a bit complicated, as I don't think you can split your library over multiple volumes.
That's a little bit over your budget; costs would be less if you have 1TB or less to archive/backup. It will give you five copies of your data -- four local, one offsite. You can save a bit by not using a RAID drive as your backup drive -- that's three local copies and one offsite. Personally, I try to allow for as many points of failure as possible.
I like the WD My Studio RAID drives for a couple reasons. One, they include a software application that lets you know if a drive has failed. Second, they are warrantied for three years, and WD will advance ship a replacement drive to you if it fails under warranty.
If your budget expands, your options do. You can look into something like a 4-disk RAID enclosure that lets you add your own storage and swap drives out as needed. There's also the Drobo, which is a proprietary RAID format. I had a network version for a while that I used as my backup drive, but it stopped functioning when I upgraded to Lion and Drobo wasn't able to provide a solution. I sold it and opted for a local RAID 5 solution for backup instead.
(RAID 5, as Craig pointed out, uses a proprietary controller to write data across four drives with parity--if one drive fails, you don't lose any data; and the system only eats 25% of storage for the overhead parity data, versus 50% for RAID 1. I wouldn't use one as my sole storage solution without backup, but I would be fine using a RAID 5 for backup and a RAID 5 as storage--again, the chances of both failing simultaneously are remote enough that I'd not worry too much about losing my data, and as always, there needs to be an additional offsite copy to be reasonably bullet proof).