Can't startup from external USB SSD

Startup in macOS Recovery > go to Disk Utility > select the startup volume on the external SSD > select First Aid > click Run. see > https://support.apple.com/guide/disk-utility/repair-a-storage-device-dskutl1040/19.0/mac/10.15

Yea you need to do it from macOS Recovery.

Then if that doesn't help?

Try starting up from the internal drive > go to the Startup Disk preference > re-set it to startup from the external drive and click Restart. see > https://support.apple.com/guide/mac-help/change-your-mac-startup-disk-mchlp1034/mac

Running disk utility from recovery mode was no help. I have the external SSD set as the startup drive in system prefs. I have since tried 4 other external drives that will work as a startup drive on other computers, but are not recognized by this imac after the system update. The drives all work fine to start up other macs. I have tried drives on all of the USB ports with no difference. As soon as the imac start's up, any of the drives are recognized and function properly, I just can't startup from them.

Prior to installing system update 2020-007, this imac had been starting up from the external SSD just fine for the last couple of years. Upon restart as part of the system update installation, the imac reverted to starting from the internal (now backup) drive and will not recognize an external drive until after startup. The external SSD is still showing as the startup drive in system preferences. Upon trying to restart the computer holding down the option key, no external drive appears, only the internal HD and the recovery partition.

I have of course tried resetting NVRAM & SMC with no effect.

I'm at a loss.

We have almost exactly the same setup with our late 2013 iMac and have tried all the same suggestions...resetting NVRAM, starting in recovery mode....we did find that the external showed up if we had unplugged it before starting in recovery mode and then plugging it it...it was recognized and selected, but wouldn't boot and defaulted once again to the internal macHD.

I've seen elsewhere on the net several mentions of the late 2013s being affected by this security update. Also looking impatiently for a resolution.

I‘ve just made the same experience with two different external USB SSDs. If any was plugged in on start or restart the selection menu for startup disk would not appear (option key ignored) and after quite some delay (much more than usual) it booted from internal HDD.

The only difference is that when I unplug all USB devices then option key works and when my internal drives appear (I have a Windows bootcamp partition as well) I pug in the SSD and it appears after a few seconds and can be selected as Startup disk and booting from it works fine.

That one SSD (SanDisk Extreme Pro Portable SSD 1 TB) booted slower than the HDD is another story. The other one (Crucial X8 1 TB) is superfast fortunately.

I've run disk utilities on this drive multiple times, but I'll try again from recovery mode.