Can't boot from SSD following security update

[maythyme]

For some time I have been booting my late 13 iMac from an external SSD. I found that the SSD gave hugely improved speed and performance. I run Catalina on the SSD but have kept High Sierra on the Hard Disk for occasional use with older applications.

After installing an Apple update this week – I think it was a security update, my Mac will no longer boot from the SSD and reverts to booting from the Hard Disk. In the startup preferences the SSD is shown as the boot disk but the machine will only boot from the Hard Disk.

I have tried restoring both the Hard Disk and the SSD from Time Machine backups. I have also reset the PRAM and SMS but to no avail.

If I hold down the ALT key on startup the usual menu of startup disk options doesn't appear and the machine continues to boot into High Sierra on the Hard Disk.

If I disconnect the SSD and restart holding down the ALT key the menu of startup options DOES appear. If I then plug in the SSD it appears as a startup option and if I select it the Mac finally boots up from the SSD. Everything then works fine but if I shut down or restart I go back to square one and the machine boots from the Hard Disk once more.

Now despairing! Any suggestions much appreciated!

 

[Lufbrarunner]

Are you able to select the SSD from  >System Preferences > Startup Disk ?

 

[Cory Cooper]

Hello and welcome.

I assume you mean you installed a security update on the external SSD running Catalina, correct?

C

 

[maythyme]

Yes, it's selected in the system preferences startup disk. I have also tried setting the startup disk using Terminal but it doesn't make any difference.

 

[maythyme]

Hello and welcome.

I assume you mean you installed a security update on the external SSD running Catalina, correct?

C

Yes, that's correct.

 

[Cory Cooper]

OK, thanks.

I wonder if it could be a Security Preferences issue?

-What brand/model is the external SSD enclosure?
-Does it have any form of security - thumbprint reader, etc.?

C

 

[maythyme]

It's Seagate OneTouch SSD. No form of security.

 

[Cory Cooper]

OK, thanks.

Looks to be compatible...there is an article on Seagate's site if the drive is not recognized:

USB External Troubleshooter

Maybe that will help?

C

 

[maythyme]

Many thanks! Will check that out.

 

[Cory Cooper]

No worries...let us know if that works.

C