A better term for this might be “image retention strategy (in case of disaster)”.
There are 2 types of contingencies that need to be accounted for.
The first is where the hardware on which you store your images goes away. It breaks. It is stolen. It is burned up in a fire.
The second is where you initially decide that you really didn’t want that image, or want it the way it is and decide to change it. And then 3 weeks later change your mind.
The core of my backup strategy involves Apple’s Time Machine to an external FireWire 800 drive. This covers both contingencies to a point; it covers the case where a hard drive fails, but not where the backup (Time Machine) drive goes missing also. It is called “Time Machine” because it offers a journaling (or a versioning) strategy allowing you to recover states of a file in case they change over time (to a point – there is a rollup that occurs from hourly to daily to weekly to monthly).
But let me back up (pun intended) for a second.
My image storage policy involves 2 areas, both online. I have an iMac which unfortunately only (currently) has 1 internal drive. In a perfect world I would have 3 internal drives: one for the OS and applications, one for Photoshop’s scratch area, and one for photographs. As it is, on the one system disc all 3 are hosted.
However, the images are what I would call “in progress” images – the ones I am currently working on. The disk on which they are housed is backed up with Time Machine to the aforementioned external FireWire drive.
I also have another external drive on which my “Photo Archive” is stored. Although I occasionally return to edit these, this forms the second part of my storage plan. By moving these here, it frees up space on my main system disc and allows me to move them to a relatively static environment. Discs that are actively used need to have some extra disc blocks available as the OS manages the disk space. Relatively infrequently accessed discs can use more of their capacity for storage; this is an added benefit to this plan.
As I use Lightroom to access all of my images, the fact that 2 physical drives are used to store images becomes relatively transparent.
Ah, but there is a little more to the story. I use BluRay discs to enable off-site storage of images. So what I do is collect shoots into 25GB “chunks”, make 2 copies of the shoots (folders) that comprise these “chunks”, and then move them over to the Archive disk. Oh, and the Archive disc is also Time Machine backed-up to that same external drive holding the system disc’s backup.
So the result is that images stored on the Archive drive are actually held in 4 places: the Archive drive itself, the Time Machine drive, and 2 BluRay discs, one of which finds its way offsite.