External HDD Drive Mounts But Will Not Boot
Problem: iMac will not boot from external USB 3.0 drive. My main system is a 3.5 TB volume that is on the external drive (named “Crescendo"). The problem began when an unknown malfunction caused the Mac to restart. It then booted from the iMac's internal drive. On the iMac’s internal drive desktop screen, the external drive shows up on the desktop and in the startup disk window. However, there is no OS listed under Crescendo’s icon in the startup window in system preferences. Whenever I select the external drive as startup it glitches when attempting to boot and defaults to booting from the internal drive again.
Thinking that perhaps the external drive had failed, I bought a brand-new replacement and proceeded to restore from a Time Machine backup. This also failed at the last step and it booted up from the internal drive again. I then suspected maybe the drive enclosure was corrupting the operating system. So, I reformatted the disk, bought a replacement USB 3.0 enclosure and tried restoring in the same way again. Same result: The external drive shows up on desktop and startup disk window, but no OS is listed and the computer will not boot from the external drive. I have tried disk utility/first aid in Recovery Mode. I have also tried reinstalling OS in Recovery Mode. No matter what I’ve tried the computer will not boot from
the external drive. I zapped my NVRAM, too. None of these have solved the problem. Any help is greatly appreciated. Please see attached screenshot of Startup Disk window in System Preferences.
iMac 21.5″, macOS 10.15