Migrating MacOS Monterey 12.7.6 from iMac to Mac mini's external SSD and making it a boot disk

The Mac mini M4 shipped with macOS 15 Sequoia when it was first released last year.

This is why it cannot run macOS 12 Monterey.

No Mac can run a version of macOS that predates its own release date.

You may be able to install macOS 15 Sequoia on that Mac mini, but nothing older.

As others have stated, the M4 Mac Mini does not support Monterey for booting.

Short of the annual expense of running Monterey as a guest on Parallel's Desktop on that M4 Mac Mini, you might get the free copy of VMware 25H2 and install Monterey as a guest on it. Once that is done, you could mount a Monterey Time Machine drive to that Monterey guest and use Migration Assistant to transfer your Monterey specific content. If this works, that would give you a M4 Mac Mini running Sequoia or Tahoe and a means to boot into Monterey as you needed it.

Of course I haven't tested the previous paragraph as I have Monterey as a Parallel's guest on my M4 Mac Mini Pro. I can say that Monterey boots up as a Parallel's guest pretty darn quick on ARM hardware.

User wrote “ 2 TB SanDisk APFS external SSD boot drive with MacOS Monterey 12.7.6

“ I connected a 2 TB KingSpec external SSD to the MacMini. It's now APFS formatted but contains nothing (and we'll call it the DESTINATION drive). “

“ Migrate the contents of the SanDisk SSD (SOURCE) to the KingSpec SSD (DESTINATION); “

“ Make the DESTINATION disk a BOOT DISK; “

Cloning from 1 External Drive to a second external drive maybe possible 

Have done that several times using Carbon Copy Cloner 

You just just need to assigned the Source as SanDisk APFS external SSD boot drive with MacOS Monterey 12.7.6 and the DESTINATION  as the KingSpec SSD

But as others have rightly mentioned several time

The  KingSpec SSD will never be bootable on the M4 Mac Mini 

As others have stated, the M4 Mac Mini does not support Monterey for booting.

While it wasn't clear in the opening post, acemguar clarified that point: Migrating MacOS Monterey 12.7.6 from iMac… - Apple Community

You wouldn't be able to boot Monterey on an M4 Mac, even if you could create an external boot disk.

2) Install Tahoe on the Kingspec SSD. Good luck. People are finding it difficult to install on an external drive.

1) When you boot from the Kingspec SSD, use Migration Assistant during setup to migrate your data.

Well, its a slow Saturday and I decided to work through the VMware 25H2 product installation and Monterey guest scenario on my M4 Mac Mini Pro.

Downloaded the free VMware Fusion Pro 25H2 and installed it. Unfortunately, it throws a missing file dialog near the end of its installation on Tahoe 26.0.1, and until Broadcom issues a fix, it is done on Tahoe. I will try again once 26.1 is released, or Broadcom issues an update. VMware Fusion Pro 13.6.4 also failed with the same message.

VMware Fusion Pro 25H2 does install and work correctly with Sequoia (Intel) as I have VMware guests running on Sequoia v15.7.1. The VMware installer is advertised as supporting Intel and Apple Silicon hardware… 🤯