I have had a two-month nightmare with Mojave and my late-2014 iMac 27”.
Mojave wouldn’t install on my OEM fusion drive - found problems - so I installed a SATA SSD from OWC. Still problems. Took to apple store. They identified a bad RAM module and installed Mojave for me. Replaced the RAM.
The machine was never reliable thereafter. Ran OK for short intervals before going into molasses mode for most of the time. Disk utility wouldn’t even make it through the volume. Tried to reinstall Mojave and made it worse (woudln’t Complete the process). Eventually it woudn’t boot any more, after hours logged w/apple support.
Was back at apple store yesterday, and the techs there made this jaw-dropping suggestion:
This iMac is designed to work with a fusion drive. So you MUST replace the original HD with an actual HD, not an SSD, and fuse the drives. This seems nuts to me, as you should be able to run from any legit volume — USB external, whatever. Anyone heard this restriction before? Any logic there? Thank you....