I use multiple hard disks as well as Apple's Time Machine. I store many photographs, and sometimes have been asked for images that are 5- 6 years old, so Bill's idea of not being concerned about them just won't work for me. For example, National Geographic asked for a 5 year old photo of a Canada Goose. I would have hated to not have a copy, and would have been out of a fair amount of $...
I back up to a pair of hard disks, and when they are near full, transfer the data to a pair of larger drives, and put the old ones on a shelf (one of which is kept off site). I'm currently backing up to a pair of 4TB drives.
