iMac won’t boot from external SSD

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Level 1

19 points

iMac won’t boot from external SSD

I’ve been booting our 2013 iMac from an external USB SSD; the internal Fusion drive is flaky and I’ve associated it with repeated crashes. But after installing Catalina Security Update 2020-001, the external SSD is no longer recognized at boot time. If I restart while holding down the Opt key, the startup manager shows only the internal Fusion drive. I can still boot from the Fusion drive (though it’s not reliable for regular use), and when I do, the external SSD is visible and can be mounted. I’ve tried loading additional drivers from the startup manager (Opt-Shift-Cmd-Period) and resetting NVRAM (Cmd-Opt-P-R), but I still can’t boot from the external SSD. It was working fine until yesterday. Can anyone suggest another solution? Thanks in advance!

iMac Line (2012 and Later)

Posted on Dec 15, 2020 11:19 AM

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Answer 1

Top-ranking reply

Old Toad

Level 10

210,407 points

Dec 15, 2020 12:09 PM in response to Epiktistes

Try the following a couple of times in succession: Reset NVRAM or PRAM on your Mac - Apple Support . Then see if you can boot from the EHD.

If that fails boot into Safe Mode by booting with the Shift key held down and reboot normally. Then test to see if the EHD can be recognized as a bootable drive.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following;

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts

• Deletes font caches, kernel cache, and other system cache files

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Answer 2

Epiktistes Author

Level 1

19 points

Dec 16, 2020 9:33 AM in response to fototypo

Thanks, that's exactly the problem I was having. It's always nice to know you're not alone.

I finally got the update installed on the external SSD by rebooting while holding down the Opt key. The Startup Manager didn't show the external SSD as an option until I reset NVRAM several times, and also tried moving the SSD to another USB port. I'm actually not sure what worked, but I finally got the update installed and am running from the external SSD again, after wasting more than half a day.

Appreciate the pointer!

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Answer 3

fototypo

Level 1

30 points

Dec 16, 2020 12:41 AM in response to Epiktistes

Have a look here:

https://discussions.apple.com/thread/252177156

Some people have similar problems. No solution yet.

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Answer 4

Epiktistes Author

Level 1

19 points

Dec 15, 2020 2:10 PM in response to Epiktistes

Thanks! I had already reset NVRAM twice (as mentioned in the original post), but I reset it a third time, and the external SSD appeared in the boot menu. Go figure.

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iMac won’t boot from external SSD

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