Hi,

iMAC 27" GPU died, before it died managed to use disk util to backup the whole HDD to a .dmg file to a 1TB Hard disk.
Before that, though was OS issue proceed to reinstall but it failed a few times then I erase the hard disk

iMac is back after GPU repaired and they install High Sierra. Took back and tried to restore from recovery disk utility but after restore it won't boot.
What can I do? In High Sierra, I can view the content of the .dmg files that I back up and even do a migration when I mount it but not all software is compatible with the new OS.
How can I restore the .dmg files to the older version os?

thanks.