iMac 27” (2011) SSD Upgrade

Hi,

I have an older iMac that I’d like to upgrade with an internal SSD, but I’ve run into a weird problem. I bought a new Crucial mx500 1 TB SSD and installed it, but I wasn’t able to install High Sierra on it. The installer would always hang at the end. Eventually, I determined that the drive was not passing the disk first aid check. (It gave an exit code of 8, even after successfully being formatted as HFS+.)

Thinking it might be a problem with the new SSD, I removed it from the iMac and installed it in an external USB enclosure. When it’s connected that way, it can be successfully formatted, and disk first aid reports no errors. I then installed High Sierra on it and was able to boot from it with no issues at all. So now it doesn’t seem to be a problem with the SSD but rather some strange issue that only appears when the drive is connected via SATA internally. Weirdly though, I’ve never had any problems using a spinning hard drive with the same internal SATA connector.

Any suggestions would be very much appreciated.