I may have made some progress on this total nightmare. This probably won't help everyone, but it's a new approach that I haven't seen written up anywhere else and it seems to have solved my problem so far.
My symptoms - total nightmare for the past two weeks, began maybe 2-3 weeks after upgrading to High Sierra:
- Repeated spinning wheels of death, slowdowns, freezes which all appear to be related to difficulty accessing a hard drive. It appeared that several of my disks were damaged, as it seemed like my system was stalled as if the drive was repeatedly trying to access a bad sector. In reality, one or two of my disks appears to be truly damaged but others ended up OK.
- The random "ejects" of external hard drives that others have reported, accompanied by error messages about the drive not being properly ejected. Interestingly, this would often occur simultaneously for two drives that were mounted in the same multi-bay enclosure. More on this in a moment.
- Time Machine started having problems with disk errors, spinning wheels, etc.
I tried all kinds of things -- bought two new drives, bought a new 4-bay enclosure, new cables, and nothing solved the problem. As others have noted, it seemed like disks kept failing, even new ones. At some point, I realized that every failure was on a drive that was mounted in a 2-bay or 4-bay enclosure (non-RAID, in my case; just independent disks mounted in the same enclosure). And I then went back and read this and other threads and noticed that many people who were having problems were also using enclosures.
THEORY #1: I thus came up with the theory that High Sierra has a bug that possibly appears only when two drives are mounted in the same enclosure. And, from my observations, it crapped out when both drives were being accessed at about the same time (in other words, it ran for hours just fine if only one drive was being used, but as soon as the other one came into use, the problems began).
To test, I had two drives (6TB and 2TB) mounted in the same 4-bay enclosure (2 bays were empty) that was consistently failing with eject errors. I moved one drive to a separate 2-bay enclosure, plugged into a separate USB port on the back of my iMac 5K, and kept the other drive as the sole drive within a 4-bay enclosure. Since making that change, I have gotten much much farther without errors, doing the same things I was doing previously that resulted in an "eject" or other failure. So, for the moment, having no more than one drive mounted in any multi-bay enclosure appears to have solved my problem.
I did find another post related to enclosures.
THEORY #2: I'm not sure about this one, but I came up with it a while back so I'm going to mention it here. I initially thought Time Machine was the problem, but at this point I'm not sure whether there is a separate TM bug OR if TM is crapping out because of the problem described above. However, I noticed in many postings that report TM errors under HS, people were using an encrypted TM drive (as was I when it started failing). I'm also guessing that the TM preference file gets clobbered once you start having problems with. So, when I get brave enough to try TM again, I'm going to do a full TM reset (trashing preferences) and use an unencrypted drive. In all that I've read about TM problems under HS, I haven't seen anyone try an unencrypted drive along with a TM reset.
I'll be interested in hearing if this is helpful to anyone.
