I have an iMac (27-inch, Mid 2011) with a 2.7 GHz Intel Core i5 CPU, 16 GB of memory. It's running macOS High Sierra. I recently upgraded it to an OWC 1 TB SSD drive, and rebuilt it using a Time Machine backup from the original HDD.

It's now occasionally locking up and creating an incredibly loud, repeating sound (digital, not mechanical) from the speakers, and requires a hard reset in order to recover. It can be days or weeks between this lockup event, and so far I haven't noticed a pattern about what I might be doing at the time when this happens. It has happened once when the iMac was in sleep mode. In between crashes, the system seems to run normally.

Has anyone else experienced this, and probably more importantly, has anyone got any pointers, suggestions or ideas about how to solve this problem?

Thanks in advance!!


  • Does it exhibit the symptoms in Safe Mode? Since you have a TM backup, have you tried wiping the drive and starting with a clean install? – Allan Jan 1 '19 at 20:20
  • I haven't tried Safe Mode. When I installed the new SSD, I erased it, installed the OS and then used the TM backup to my stuff back. Are you suggesting I do that again? – writes_on Jan 2 '19 at 16:07
  • Start with Safe Mode. See if it goes away. There's no harm in wiping again, but prior to transferring your files and settings, see if the problem persists. – Allan Jan 2 '19 at 17:01

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