I have an iMac mid-2011 21.5 inch where it worked well up to and including Sierra. I always boot off of a LaCie Rugged Thunderbolt SSD 500G external drive. I have Yosemite on the stock internal where I can boot from as well if I set it as startup disk.
I upgraded to High Sierra and the iMac started to freeze (mouse, etc) and becomes unusable requiring hard boot. After a few freezes I booted off the internal Yosemite HDD and reinstalled High Sierra after wiping the external SSD (ech).
Booted back on to High Sierra/SSD and froze again after a few short minutes. I wiped and reinstalled High Sierra twice more. Same result. I.e. no 3d party apps could be involved.
I downgraded to Sierra and everything works fine and back to normal.
- NOTE 1: My hardware config is identical for both Sierra and High Sierra (i.e. external SSD). However, High Sierra only freezes.
- NOTE 2: The High Sierra SSD was not shown as mounted when booting off the stock Yosemite (Disk Utility). The SSD also wasn't mountable (I tried everything I knew--diskutil, Disk Drill, etc). The drive only mounted once I reinstalled High Sierra--although, again, it froze repeatedly once booted off the high Sierra external SSD.
- Note 3: Since High Sierra was reinstalled on an erased and clean SSD, and still froze, the notion of 3d party app interference is not applicable.
I am wondering if Sierra has become my final macOS upgrade.
Thoughts greatly appreciated.