Chosen Solution
I have a 1TB from an 2014 iMac that died. I have put the HD into an external enclosure in order to migrate to new computer. Unfortunately, the HD will not mount on the new computer. Running Disk Utility’s Repair Disk on the external HD comes back saying that the Volume is good, but the drive refuses to mount. I’ve tried a number commands in Terminal, tried DiskWarrior and Disk Drill (which allowed me to at least recover the Volumes files) but the HD still will not mount. Help!
My bet is that you have installed High Sierra or Mojave on the internal SSD drive and it has been update to an APSF format. This format now has problems recognizing external GUID formatted drives.
