You made some excellent points which I’ll certainly keep in mind.
Like you, I never had reason to keep more than a year’s worth of backups, even when managing backups was part of my day job, but Time Machine really spoiled me. The penalty for several years’ worth of backups is very small, and there have been a couple of times when they came in handy. I still don’t see the need for multi-year backups, but I do like having them.
My system consists of an iMac with a 500GB SSD, the Drobo 5D for most of my data, and a USB drive for Time Machine. The Drobo has two partitions; a Time Machine partition, and a regular partition. I back up the regular Drobo partition with Time Machine also, and the Time Machine backups stored on the Drobo are the ones I’ll keep for a long time, but I want the second Time Machine backup in case the Drobo hardware fails. I just keep those USB drive backups for a few months, so I don’t need a large drive.
Yes, the Drobo 5D volumes seem to look like regular volumes to Disk Utility. They verify without a problem, but I haven’t risked doing a repair…
They dismount and mount automatically after a reboot without the need for the Drobo Dashboard. In fact, a mount/dismount using the Drobo Dashboard can no end of grief for Time Machine, so I only do it when updating the Drobo software/firmware.
When the Drobo drive mounts after being dismounted via the Dashboard, it seems to mount with a different UUID than it had when it was dismounted, and Time Machine seems to roll the UUID into whatever it uses to identify backups. As a result, Time Machine does a deep scan of the Drobo volume, backs up everything, and it totally ignores my exclusion list (I assume that TM assumes that the exclusion list applies to a different volume).
I have a LOT of stuff on my Drobo that doesn’t need any more protection than the Drobo provides; iTunes media, for example, which I could re-download if needed. This stuff could fill up a TM backup drive in no time, so the exclusion list is rather important. If I manually dismount the Drobo volume, I have to delete then recreate the exclusion list, and manually purge the junk from my backups if I forgot to disable Time Machine before the dismount.
Things can be a bit exciting even if I let the OS dismount the Drobo volume, although I don’t think this causes long-term problems. The very first time TM runs after a reboot, I can get one or more, “Property list invalid for format: 200 (property lists cannot contain NULL)” errors. As best I can tell, after a lot of Googling, this error usually occurs when there is an item in the TM exclusion list that points to something that doesn’t exist. I don’t get it during the second, or subsequent, backups.
I’ve been in touch with both Drobo and Apple support, and the problem was escalated to second level support by both companies, and both support groups have tried to be helpful. The consensus seems to be that TM is trying to scan the Drobo volume immediately after the boot, and before The OS, Spotlight, and friends have finished with it. I’ve seen similar strange and wonderful behavior on a regular disk when I tried to access it while Spotlight was re-indexing it.
My solution is to disable Time Machine before shutting down the system, then wait a few minutes after a reboot before enabling Time Machine. I never have a problem if I do this.