Chosen Solution
I just installed a SSD kit I bought from iFixit (1TB Crucial SSD, IF174-031-3). I booted from an external drive and ran DiskUtility. Erased drive and formatted as APFS/GUID. Then installed macOS High Sierra. Install ran fine. Tried to reboot from SSD and all I get is the flashing question mark. iMac still boots fine from external drive. Any ideas here?
You’ll need to go back and reformat the SSD this time GUID journaled file system (HFS+) as your systems firmware was not properly updated to support GUID - APFS
