Some slight clarification here. RAID arrays are not in themselves a "back-up" they are protection against hardware failure and are used for performance or uptime assurance reasons. They will not help you if you accidentally or someone maliciously erases something important, can be a pain to maintain and a nightmare to migrate. I had to recover a friends RAID 5 array on an NVIDIA motherboard and the only way it could be done was to find an old board on
ebay and build up a complete machine. Software RAID under windows or Linux for non-performance critical applications is much safer as any old computer will do.
In the domestic situation, if you have 2 hard drives, you are usually much better off from a security point of view using one for your PC and using the other as an archive using software that takes a base image and periodically automatically takes deltas (changes) in important file types e.g. documents in your "my documents" folder. To this extent the Apple Time Machine or the software that comes with Western Digital drives or any number of alternatives makes more sense in most cases.
I would also strongly recommend for external connection using only the smaller 2.5" drives because these are usually derived from notebook technology which is more tolerant to being moved and knocked about than 3.5" drives. Unfortunately these are more expensive and lower in capacity. I had some really horrible experiences with IBM drives during the Deskstar ("deathstar") era and multiple failures of maxtor drives about 2-3 years ago. Since then I've used Samsung (which I beleive bought IBMs hard drive business) and not had a single problem (ironically these are also amongst the cheapest).
There is of course a very simple solution to backing up photos. Just leave them on the SD card and store these securely, I put them in a photo album and keep them in the basement which is dry and has a very constant temperature. I get about 500 on a 2GB card which I bought at around £4 each. Flash memory is generally believed to be more stable than either tape or disk. I have electronic projects from 25 years ago that have program data completely intact in flash. You also need a bit of common sense. If you have a compact camera (like my digital Ixus), you really aren't getting 12 megapixels of resolution through those optics onto that sensor. If you have anything less than a fairly decent "prosumer" camera then don't bother going over 5 mega pixels otherwise you are archiving gigabytes of noise and often interpolated rubbish. Also turn-off any in-camera sharpening or other effects and keep the source as "pure" as you can. Post-process in software as required.
As many of you will have discovered, optical discs are rubbish as a long term archive. I have no failures with TDK discs (around 100 from 2004-2007), but hit and miss with others (including many big brands).
And yes I will say it: Ultimately really, really critical data should be backed-up off site but then t'was always so e.g. using bank vaults or filing documents with a solicitors office. In the medium term, network storage will become large/cheap enough and the game will change but I still see this as a supplemental service for the forseeable future instead of a complete alternative. The big issue is more likely trust rather than techical.
