OK, Patrick,
I didn't intend to have a massive disk farm, I just repurpose drives when I have them. I have lost pictures because of a failure of my internal drive and a simultaneous failure of the backup drive, so I have now spread the pictures to 4-5 drives and three locations so that the risk is very small of that happening again.
Overall, I have 26.5TB of storage to store about 8 TB of data, not counting backups and duplicates. But drives are not that expensive these days, so I tend to collect them and leave a lot of open space on them for speed. The only really full drives are the TM and CCC backups, but each of them automatically manage space, so periodically they purge older backups to make room for newer. I don't partition drives because it gives a false sense of security to see two icons and think you have duplicates. But anything on one physical device is not duplicated because a hardware failure will lose all partitions. So all drives are single partition and usually single purpose (photos, music, videos, business files, etc) with backups and sometimes duplicates on other drives.
In addition, I have an old iMac that runs my home automation and has two external drives, one for CCC clone backup and the other for TM backups of that iMac. I don't want to have to rebuild the home automation if/when that iMac fails, so the two backups should let me recover to a replacement system without too much delay or hassle. Given that my home automation controls lighting both inside and outside my house on a daily basis, I don't want to have to sit in the dark while I rebuild the system! I have a recently retired 2011 MBP in the wings, with the HA software installed that just needs to be restored from one of the backup drives to take over for the iMac when the need arises. Basically, I treat my home systems like I did when I was running data centers for a living--never want to be out of service, ever!
Basically, every file I own (not counting the system files) is duplicated and backed up twice. Yes, that's four copies of each. And for the photos, there are 5-6 copies of them. Data is very ephemeral and the pictures I lost were irreplaceable.
Failing to prepare is preparing to fail. I don't want to fail again.