Chosen Solution
Hey folks, As per title, the 2.5” Samsung 860 EVO I installed in my late 2015 27” iMac is not being detected in disk utility. The following has been done: OWC inline sensor is in the computer. I tried plugging in the original HDD through both the original connection and through the OWC inline connector and both still manage to boot, so it’s not the sensor.SSD is detectable when placed in my external hard drive caddy and has been formatted. Funnily enough I even managed to install OSX on it and boot from it via the USB connection. After this, I connected it to the SATA connector and also through the OWC sensor; neither were detected. I have tried both Extended Journaled and APFS formatting (via the external hard drive caddy).diskutil list in Terminal does not detect the SSD when connected via the SATA connection but does (obviously) detect it when plugged through the external hard drive caddy.I have tried connecting a couple of different mechanical drives to the SATA connector - some can be detected but some cannot. I have tried connecting a different SSD which I also managed to format to APFS via the external hard drive caddy but it is also not detected.I have tried using both High Sierra and Big Sur USB boot drives as well as El Capitan from internet recovery; none can detect the SSDs. What’s really frustrating is that at the start of it all, it did see the Samsung SSD and format via SATA, but the (High Sierra) installation had an error and after that the computer just refuses to see it or any other SSDs. Somehow this error seems (outwardly) to have destroyed this computer’s ability to see SSDs via SATA, but kept its ability to see a mechanical hard drive. Is there some sort of firmware update I need to get it to see SSDs? Update: not sure if the images worked but these are the two drives in question from disk utility in the 5k iMac, with the mechanical drive connected. The USB port detects the SSD but when I try to connect through the SATA port the computer detects nothing. From what I can tell from the details on disk utility, there should be no reason why the iMac can’t detect the SSD via the SATA connection, right?
Sounds like you’re facing the HFS+ - APFS volume confusion issue! Your iMac original drive was setup with Sierra or older so its HFS+ based. Now you’ve added the wrinkle of formatting and setting up your new drive from a different system which is High Sierra or newer so it was setup with APFS. But! Your system has no ability to boot up under APFS as its EFI firmware was not updated with the needed additional services. So you need to put the original drive back in and then run the OS updater to High Sierra or newer so the systems EFI is updated. Or… using the older OS use Disk Utility within it as it too was updated which is why your older version failed to see the drive, and just stick with it.
