Some people may be wondering why I ever added a 3rd drive. Drive 1 was needed, of course. I added a 2nd drive in order to have a readily available backup in case the 1st drive failed. I then decided to add a 3rd drive so that I could store it off site. I did this several years, OSes ago. However, what I noticed then, was that Time Machine would work fine for a couple days, and then would start telling me that there was a problem w/ one of the drives (not true). I'd disconnect that drive, and Time Machine would tell me that there was a problem w/ one of the other 2 drives (again, not true). I'd disconnect that one and shortly later it would tell me that there was a problem w/ the last remaining drive, (yet again, not true). I'd reboot, reconnect all 3 drives and things would be good for a few days, and then ...
So I decided to disconnect the 3rd drive and wait for the next OS upgrade. That didn't solve the problem, so again, I disconnected the 3rd drive and waited for the next OS upgrade. The 3rd drive has been disconnected for a long time now. My assumption was that if Time Machine couldn't find the drive, it would use 1 of the other 2 that were available. But then a few months ago, I caught Time Machine backing up to that 3rd drive, which hadn't been connected in over a year. I never got any kind of error message from Time Machine telling me that it couldn't do the back-up due to the drive not being available. Hence, the reason for asking my question. Now, I understand about local snapshots. I'm now thinking that since it's been a while since the 3rd drive was connected and there have been several OS upgrades, maybe I'll reconnect it to see if Time Machine can now handle more than 2 drives.
Thanks all who responded.
