Error Message 'You may not install to this volume because the computer is missing a firmware partition'

Question

Level 1

27 points

Error Message 'You may not install to this volume because the computer is missing a firmware partition'

My Late 2015 iMac 17,1’s internal SSD failed and no longer shows up in Disk Utility.

I have gone into Recovery mode and installed El Capitan on an external SanDisk Extreme 500GB SSD to boot up the Mac. I cannot install past Sierra as I am met with 'You may not install to this volume because the computer is missing a firmware partition'. How do I get/install this firmware partition?

What is strange, I first did this with a WD My Passport hard drive and was able to get it to Monterey, which is the last macOS supported, but I can’t get the SSD which has decent performance compared to the WD hard drive. Both drives connect through the USB-A ports. I have correctly partitioned the SSD via GUID via the system.

Yes it is old, but everything else is working and it's good for basic tasks.

iMac 27″, OS X 10.11

Posted on Sep 22, 2024 2:40 PM

Reply

Answer 1

HWTech

Level 9

60,758 points

Sep 22, 2024 7:43 PM in response to GGHulse

Later versions of macOS require a working & properly formatted internal drive when installing macOS to any drive (even an external drive)....the internal drive can be empty though, but it must be properly formatted using Disk Utility so it has a hidden EFI (aka ESP) partition and a standard partition with HFS+ or APFS (depending on the OS being installed).

Is your external drive properly formatted using Disk Utility? Some third party external drives ship with odd partition layouts & file systems....sometimes with hidden things as well. Here is an Apple article on how to erase the whole physical drive which will make sure to create a brand new partition table including the hidden EFI/ESP partition. If your external drive does not have this hidden partition, then that may be the problem.

Erase and reformat a storage device in Disk Utility on Mac - Apple Support

Just curious on the exact model of your iMac? You can get this information by clicking the Apple menu and select "About This Mac". And what is the BootROM version (this can be found in the Apple System Profiler.....Option-click the Apple menu & select the first item). If the system firmware is not up to date, then the later macOS installers will want to try to update the system firmware.....that process definitely requires a working & properly partitioned & formatted internal drive....with macOS Monterey installer, that internal drive must be an Apple OEM drive).

Reply