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.

  • Just curious because I might have similar issue: Do the freezes occur in short bursts (like 2 seconds) and then give back control, or is the computer completely stuck indefinitely? If you are playing audio when the freeze occurs, does the audio continue playing but the mouse is stuck? – Winterflags Jul 10 '18 at 20:48
  • Indefinite freeze. I have waited but never experienced unfreeze and normal operation. I am doing nothing but surfing web (no other apps like music, video, etc). – MoMo Jul 10 '18 at 20:54

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