Chosen Solution
Dan, I am trying to replace the 1TB HDD on a Retina 4K Display (2017 iMac 21.5" which has a 1TB Fusion Drive setup to a Crucial 1TB SSD. Booted into Recovery Mode to use Terminal commands to split the two drives into a 28GB blade SSD and a 1TB HDD Opened up the iMac and replaced the HDD with the 2.5" SSD Booted the iMac from a macOS 12.5 installer drive and tried to format the SSD using Disk Utility. But whether I formatted it as HFS+ or APFS, it would fail validation (via First Aid) To test if it was a bad SSD, I connected the SSD to another Mac running macOS 12.5 and reformatted the drive successfully as either HFS+ or APFS. I then installed macOS 12.5 on the SSD and booted the Mac from the SSD. I also booted the iMac from the SSD in an external enclosure. So I know I have a good SSD that WILL boot this iMac However, when I connected the SSD to the iMac’s internal 2.5” bay it would not “see” the above SSD and obviously would not boot — However, it DOES “see” the original 1TB HDD Why is this iMac have a problem with this SSD?
Have you ever gone to the throne room and exited with a TP tail? Humor aside, you forgot to take the blade SSD out! Apple loves altering things. The terminal command often doesn’t work depending on the macOS version and the updates applied. I haven’t spent the time to dig into this mess as I don’t fix that many systems anymore. Follow this guide iMac Intel 21.5" Retina 4K Display (2017) Blade SSD Replacement
