There are two external hard drives. The hard drives are mirror copies of each other. (or they're a two-drive enclosure)
The hard drives are stored in protected locations.
The drives aren't used for anything other than backups. Casual sneakernetting must NOT be done on these.
The list of items that are backed up (possibly including complete file lists), and the date of each backup, is stored in text files in a Git repository. The git repo assists in tracking when items are removed or added, and helps ensure that manifest data is never lost.
The manifests are usually accessed from a copy residing on an easily-accessed computer.
However, the manifests are also stored on each hard drive, to prevent loss.