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I have a 13 inch MacBook Pro 2015 model. The Mac had Big Sur installed on the internal SSD. This internal SSD died and thanks to macOS Recovery over the internet (Option-Command-R), I managed to install El Captain on an external SSD drive.

I am trying to upgrade to Big Sur, but the option to choose my external SSD during installation is greyed out with the message

"You may not install to this volume because the computer is missing a firmware partition".

Is it possible to install any macOS higher than Catalina on an external SSD?

Below is the output from the command diskutil list.

enter image description here

7
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After installing El Capitan on your external SSD, you have to restart and run macOS Recovery over the internet AGAIN.

This time it will show you an option to re-install Big Sur. But before that, make sure you reformat your external SSD as APFS in disk utility first!

APFS option is available now since it was missing the first time you formatted your hard drive to install El Capitan.

You can now install Big Sur on your external SSD!

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  • What version of macOS was installed on the internal drive before the drive died? Also, which keys did you hold down to boot to the internet recovery that allowed the installation of Big Sur? Did you hold down different keys to boot to the internet recovery used to install El Capitan? Sep 10 at 20:05
  • My internal drive was running Big Sur. No I used the same keys as before to boot into recovery mode. Option (or Alt) + Command + R keys. If i had to guess, maybe the internal firmware updated itself while i was running El Capitan? I was honestly stunned to see the recovery mode updated after i booted into it again. Sep 12 at 2:06
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    Since you had Big Sur installed on the internal drive, the Mac already had all the necessary firmware updates to install Big Sur on the external drive. I should have thought of that before posting my answer. Like you, I do not know why macOS Recovery over the internet installed El Capitan before Big Sur. Since you solved your problem, I deleted the answer I posted. Sep 12 at 3:51

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