I can't install Monterey on my compatible iMac. I use and I boot from a Samsung external solid HD (much faster) and I use the internal HD as mass storage.
brcause of the EFI Boot ROM requirement for the monterey upgrade, the host computer needs to be running monterey before you can install monterey on an external SSD. i have tested this and it does work.
As pervious posting mentioned and there are several other person expiring similar issue as yours.
There are reports appearing on the ASC Forums regarding failed installation of Monterey on older Qualifying Apple Computers. In most cases this is related to having a NON Apple Original internal drive ?
In your case - the External Drive will be the cause.
The EFI Boot ROM included in Monterey is verifying for Original Apple Drive and if not found , may Balk or fail to upgrade to Monterey.
Reports indicate the EFI BOOT ROM is Required to Flash the Computer and is a requirement for a Successful Monterey Installation.
Thank you very much. I'll try and let you know
you're welcome. :)
and just FYI, i tested that with a Samsung T7 portable SSD.
Monterey not install on Samsung Solid state external drive