This question was asked in reply to my answer to the previous question.
When I started to back up the site I thought that I'd better make sure that if the worst happened I had a good system in place. I started with 2 ZIP-100 disks and alternated between them. Update the site on Sunday (week 1) backup to disk 1, next week (week 2 ) repeat but use disk 2.
Then I bought another drive, with extra disks, and decided to extend the disk routine to a 4 weekly rotate (by this time I had 2 other disks rotating with the photos and PDFs on). Then came the upheaval of the kitchen install so I started backing up to the external HDD as the power socket that I plugged the ZIP drive into was being used by a fridge.
Back to the reason why: If the server crashed and the PC died at the same time (taking out the current site data) I could go to "last weeks update" (on ZIP disk) and take it from there. If that disk failed at the same time than I could go back another week and so on. The likelyhood of server, PC and all 4 disks going "toes up" at once seems a bit much but, hey! that's a lot of work there and I don't want to have to do it all again!