iMac running on external SSD won't boot from it after upgrade to Catalina

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Level 1

8 points

iMac running on external SSD won't boot from it after upgrade to Catalina

For a couple of months I've been successfully running my 2013 iMac from an external SSD (for speed reasons). Initially, the SSD was a clone of my internal High Sierra Macintosh HD. Since then, program updates and content were added successfully.

Today I performed a successful upgrade to Catalina. The system looked fine.

I then performed another recommended security + Safari update. When completed the Mac booted to the old High Sierra OS from the internal HDD!

I used the Startup preferences to set the external 10.15 OS X SSD as boot disk - no effect! still booting from HDD.

I tried rebooting into recovery mode (cmd+R+power) to get access to the disk utility - no effect! the boot process form HDD continued regardless of that. This key combination worked fine when I cloned my HDD.

This is a terrible mess! I'm afraid to try anything else not to worsen the situation.

Any way to set it right? i.e. boot from the external Catalina SSD

Comments:

1) The external SSD is now displied in Finder as two disks: Macintosh HD and Macintosh HD - data. This was probably created during the upgrade to Catalina. No idea why. The internal HDD was never partitioned through all previous OS X upgrades.

2) If worse comes to worst, I have a Time Machine backup of the SSD prior to the Catalina upgrade. Not sure how to use it under the circumstances.

Thanx

iMac 27″, macOS 10.15

Posted on Dec 30, 2020 6:47 AM

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Answer 1

Dec 31, 2020 6:06 AM in response to along99

There have been several reports of this issue after installing 10.15.7 and Security Update 2020-001:

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

I also experienced this problem and had to disconnect all external USB devices, except my SSD, which I plugged into the 1st USB port (next to the Thunderbolt ports) on my iMac.

I then reset NVRM several times, and was able to boot up again in Catalina.

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Answer 2

Dec 31, 2020 6:49 AM in response to along99

Resetting the SMC is one thing that I did not try. I will keep that in mind if the problem returns with a future Security Update.

As I recall, it was the restart after installing the 2020-001 SU which caused my iMac to boot from the internal HD (macOS Mojave).

As for now, my iMac seems to be operating normally with the external SSD (macOS Catalina) since the last restart, but still shows the 2020-001 SU as "ready to install, restart required.".

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Answer 3

along99 Author

Level 1

8 points

Dec 31, 2020 6:29 AM in response to William Tomcanin

Thanks William,

Your response and link coincide with my (frustrating) experience and the information I gathered in various forums.

Anyways, as for myself I have good news and potentially useful information that I wish to share.

After half an hour on phone with Apple's support, my iMac works. For now.

The solution was SMC reset, which is: unplug all accessories, unplug from AC power for 15 sec., plug in again, wait 10 sec., and power up iMac.

I was astonished by this simple solution!

Few reservations though:

1) Maybe the other steps I performed along the way (such as NVRAM reset) also played an important role.

2) Surprisingly, now that its back on, Catalina recommends to perform 2020-01 security update. I'm sure I did that before, and if I'm not wrong, the 'disaster' happened immediately afterwards. I'm avoiding it for the meantime. Apple Support said there is no known problem with it and I could try that in future. I wonder if they even aware of the problem this security update causes to people.

3) The SSD upgrade to Catalina seems to have affected the iMac operation when it booted to High Sierra from The HDD. I'm not sure how it will perform should I need to boot again from HDD. Maybe the SMC reset corrected that as well. Who knows...

4) I'm still concerned if my iMac will continue to properly work from SSD for a prolonged period, and should I avoid future security updates, which sounds bad.

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iMac running on external SSD won't boot from it after upgrade to Catalina

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