Unable to revert to original factory OS High Sierra after upgrading PCI-Express card
I removed the Fusion drive from 2017 Kaby Lake iMac and replaced the original Apple Fusion Drive with an unformatted Aura Pro x2 500GB PCI-Express SSD and a 1 TB SSD.
Before starting I backed up all my data to an external USB SSD formatted APFS running High Sierra and checked that I could boot and run the iMac from this external volume.
Upon restart I could not boot from my external drive and ended up with a network install of Monterey.
Since then the iMac has worked flawlessly except that I have never been able to boot into High Sierra from my external drive or install High Sierra onto an internal partition.
From what I have read on line there is some EFI magic being generated by the original Apple PCI card that has not transferred with the upgrade that prevents reverting to the original factory installed OS.
I still have the original Apple PCI cardI am currently running Big Sur.
Two questions:
- Will reinstalling the original Apple Fusion PCI Express card resolve the problem?
- Is there any way of achieving this without stripping the iMac down again?
iMac (2017 – 2020)
