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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!

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  • When booted from the internal macOS on the Fusion drive, does the external macOS appear in the Startup Disk pane of System Preferences? If so, can you select to boot from the external macOS? Also, did you apply the Catalina Security Update 2020-001 to the internal or external macOS? – David Anderson Dec 15 '20 at 21:01
  • Thanks, David — good question. When I booted from the Fusion drive, the external SSD did appear in the Startup Disk pane as the designated boot disk. And it seems the problem has now been solved. I mentioned in the original post that I had reset NVRAM (twice). I reset it a third time, and lo and behold, could now boot from the external SSD. Go figure. – Diogenes Pontifex Dec 15 '20 at 22:25
  • When I boot to my internal drive on my late 2013 iMac, I do see it and can select it from the startup drives but when selected the Mac still doesn’t boot from that drive. – daneb Dec 19 '20 at 0:48
  • There’s more reports of this on Macrumors forums. This update is a disaster. I was able to get my external drive to show up on startup (and booted from it) after I disconnected another external drive. Before that I reset SMC and NVRAM several times. – daneb Dec 19 '20 at 1:08
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I mentioned in the original post that I had reset NVRAM (twice, as it happens). I reset NVRAM a third time, and lo and behold, could now boot from the external SSD. Go figure.

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  • Thank you for posting this. I just ran into the same problem. Super infuriating as the external drive was otherwise visible to the iMac from my internal drive as well as from the recovery mode screens. – daneb Dec 19 '20 at 0:37

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