External SSD no longer bootable

Solution 1:

When upgrading from Sierra to High Sierra, the firmware on older Mac logic boards is also updated. This allows a Mac to boot the newer macOS versions (starting with High Sierra) from APFS container partitions. Sierra can not be installed to boot from a APFS container, but Sierra can recognize APFS and therefore allows the selection an APFS volume with Mojave as the startup disk. However, the firmware may not agree with the selection. Possible reasons for failure are given below.

• The upgrade from Sierra to Mojave do not also update the firmware.
• The firmware update did not occur because you used an external drive to upgrade macOS.

You could backup your important data to an external drive, then do a clean install of High Sierra to the internal drive. If High Sierra is not available, then select a newer version of macOS. Afterwards, you can do a clean install of Mojave to the external drive.

Update:

Back when High Sierra was first released, I remember many questions similar to your question were posted here a Ask Different. I believe the consensus was that the firmware can only be upgraded from the internal drive. The update is written to the EFI partition on the drive before the firmware is instructed to update itself. The firmware is not capable of reading the update file from a USB drive. The firmware is capable of reading the update file from the first internal drive (disk0).

Solution 2:

Just to close the loop --

Option at boot worked when I switched the keyboard to a different USB port. I can now boot into my external drive(s) but only via the boot menu, not by setting it Startup Disk pref pane. Not perfect but good enough and thankfully a simple solution.

Solution 3:

I have a similarish issue. 2013 Mac, running Catalina upgraded from High Sierra. I could actually boot to a external SSD then a install broke that (still not sure what happened). Through recovery i reinstalled to my old internal drive (which i thought was broken), all good.

Then attempt to recreate my external drive. I tried everything to create a bootable external SSD with no luck. Everytime i tried, i could start a install but it wouldn't complete, on reboot it just went back to the login page.

I could only reach Recovery if i disconnected the external drive. I've now tried partitioning the old drive to see if that works which it does! So i can create two bootables just on the same drive. Still can't create externally though.

What was the issue with the USB ports?