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I have a Mac Mini (late 2014) with its stock internal drive and an external SSD via USB. The external drive has Sierra and this is what I always boot from. The internal is a clone of the external just for backup, cloned via SuperDuper.

According to https://support.apple.com/en-us/HT210190 I can update directly to Mojave on this Mac Mini. I downloaded the installer from the App Store and ran it to install on the external drive. It took about 25 minutes and gave a "success" message, prompting me to restart to finish. When I restarted, it booted from the internal drive instead. I went to the Startup Disk pref pane and made sure the external was selected and tried again. It booted again from the internal drive. Good thing I had it as a backup :)

In the Startup Disk pref pane, the external drive is selectable but it still shows as a Sierra boot. I can tell the install did write to the external drive but I suppose it didn't finish. I ran the installer again -- it only took a few minutes and again showed "success" and a restart prompt. Same result.

I cloned the working internal boot drive back to the previously-working external SSD. So in theory I would be back where I started, with the SSD running Sierra and being my boot drive. But it still won't boot into it and it still shows as a valid choice in the Startup Disk pref pane. I tried resetting the NVRAM but no luck. And holding option at boot does nothing -- yes, it's a wired keyboard ;)

Thoughts?

  • What exactly does "holding option at boot does nothing" mean? Nothing appears as a choice in the Startup Menu? You can choose the external drive, but the internal drive boots instead? Or, something else? It also would have been helpful to include the output from diskutil list for the configurations that do not boot properly. – David Anderson Nov 3 '19 at 6:47
  • @David Anderson No startup menu appears, it goes right to normal boot. Literally as if I'm not holding Option at all. – mistercoffee66 Nov 3 '19 at 16:20
  • @DavidAnderson diskutil output for the external in its current state (Sierra, won't boot) is/dev/disk2 (external, physical): #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *512.1 GB disk2 1: EFI EFI 209.7 MB disk2s1 2: Apple_HFS MySSD 511.8 GB disk2s2 – mistercoffee66 Nov 3 '19 at 16:30
  • Is the wired keyboard an Apple keyboard? The behavior you are describing would indicate the use of a Windows keyboard. Has holding down the option key ever worked in the past with this keyboard? – David Anderson Nov 3 '19 at 16:58
  • @DavidAnderson Yes, Apple wired keyboard and yes, option has usually worked. In fact, resetting NVRAM requires command-option and that worked. – mistercoffee66 Nov 3 '19 at 18:15
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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).

  • Thank you @David Anderson. If this is the case, why does the external drive no longer boot when restored to Sierra? – mistercoffee66 Nov 3 '19 at 16:23
  • I should add that I'm reluctant to do a clean install on the only drive that's currently bootable. Looking at my other external drives to see if any are free to wipe and install a bootable OS... – mistercoffee66 Nov 3 '19 at 18:19
  • A different drive didn't change anything (see comments to OP) – mistercoffee66 Nov 3 '19 at 21:06
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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.

  • Finding a solution can be difficult when you have more than one problem. So can I assume you can now boot Mojave on the external drive? – David Anderson Nov 4 '19 at 14:15
  • @DavidAnderson Yes, thank you for your help. I hope this last part helps anyone else w a 2014 Mac Mini to remind them that not all the USB ports are equally functional. Certainly muddied the waters for me. – mistercoffee66 Nov 4 '19 at 16:39

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