Basically tape backups but with a different medium. I'd be surprised if they didn't have any offline backups.

Tape backup is re-writable though isn't it? I had inherited one in the 90s and could re-write, granted it was painfully slow. Or are you saying that Blu-ray is re-writable too? The first iteration I used a few years ago could only write once.

Maybe disks are cheaper, even if you can't reuse them. As an added benefit, you can also restore info from the more distant past if you recognize errors too late.