After successful update to 10.15.6 on iMac w/ external boot drive, now does not boot

[CheesePuff]

I have a Late 2015 iMac 27" 5K with an external USB 3 SSD drive I use for the boot drive.

I installed the macOS 10.15.6 update this morning, it installed successfully and rebooted, after that I noticed Safari and Mail had quit unexpectedly (and Finder did not show the normal crash dialog) so I decided to reboot, and now it will not boot and quickly displays the Apple logo then a circle with a line through it. Tried PRAM and SMC reset, rebooted into recovery mode and ran disk utility first aid on it and found nothing wrong.

 

[Taz Mangus]

Boot into Recovery Mode, download the full 10.15.6 version and install it to the external drive. Maybe that will help.

 

[CheesePuff]

Boot into Recovery Mode, download the full 10.15.6 version and install it to the external drive. Maybe that will help.

That was it - I booted again with command+r and just chose reinstall macOS, took about 25 mins and then rebooted fine onto the external drive without issue (and of course with my existing data still intact). For some reason after all these years I thought that option always did a clean install.... thanks!

 

[Taz Mangus]

That was it - I booted again with command+r and just chose reinstall macOS, took about 25 mins and then rebooted fine onto the external drive without issue (and of course with my existing data still intact). For some reason after all these years I thought that option always did a clean install.... thanks!

Your welcome. Glad it worked for you. Did you verify that what got installed was 10.15.6?

 

[CheesePuff]

Your welcome. Glad it worked for you. Did you verify that what got installed was 10.15.6?

Yup, it was 10.15.6 using just normal recovery not internet recovery mode.