You mentioned troubleshooting, did it include these:

Jul 31, 2018  The machine will start to boot from the USB drive. Way 2: Set a Mac Boot from USB Drive using Startup Disk. When you use Startup Disk preferences to set Mac boot from an external drive, so it’ll boot from that disk until you choose a different one. Here is how: Step 1: Go to Apple menu System Preference, then click Startup Disk.

NVRAM reset

External Drive Bootable Asking For Update To Mojave Mac Osx Computer

Shut down the MacBook Pro. Start it and immediately hold down Command, Option, P and R keys at the same time. Wait until you hear the startup chime for the third time before letting go.

Also, SMC reset may be needed (that usually doesn't fix a missing boot partition, though).

• Shut down the computer.
• Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
• On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
• Release all the keys and the power button at the same time.
• Press the power button to turn on the computer.

Restart and hold down the D key to enter the Hardware Test. Follow the instructions.

When booted externally, use Finder's Go menu to open Utilities. Launch Disk Utility and look for any signs of your internal drive in the list of hard drives on the left. If you find it look for First Aid and try to use it on the disk.

Refer to the following to see if it takes you anywhere you haven't been yet.

External Drive Bootable Asking For Update To Mojave Mac Osx Software

Feb 10, 2017 3:37 PM