The idea is to not have the backups stored on the same hardware or even same type of hardware. Same hardware is obvious but same type of hardware is listed because if a manufacturing defect or a known vulnerability is present it would make all of your backups at risk. So you want to have backups stored on 2 desperate types of storage media. HDD and Tape, or Cloud etc...
Exactly. If tape was easier for me to implement in my setup, I would do it, but I'd rather just use cloud with a fast fiber connection for now. Offsite I can send data on an external drive to another physical location if needed.
I made efforts to buy HDDs from different sellers even, to avoid sequential failures from singular bad batches. That's something else I'd want to add to a "3-2-1", with regards to HDD as a form of backup or storage media.
for me it was always 2 different formats (db-dump, vm-dump) because i trust (good) storage-media more then backup-software, for example old veeam-backups cannot be restored with new version, old veeam-software runs not on new esxi etc...
Yes seems I used the wrong word here. Indeed I meant media, rather than format, as personally for my backup setups it's different media I want to trust, rather than the actual backup file formats (which are easily interchangeable depending on what's used; read and write data to and from formats if needed).
My 3-2-1 comes from a personal non-professional standpoint, thus not having the extra 1-0. However I have been considering immutable offline backups, using burned DVDs or Blu-Ray discs. That's another project for another time though, for now I'm trusting paid cloud providers.
As for verification tests, hashsums are a simple solution in my opinion, but I've moved to ZFS and BTRFS to avoid having blips.
3-2-1-0 Applies to all data, at all time, in all places