My wife and I have identical iMacs with the same SSD drives. On her computer a drive "PCI-Express Internal Physical Volume" shows up - it does not appear on my iMac at all. It cannot be unmounted. What is it and how do I make it disappear?
Formatting an APFS drive and creating an HFS+ partition in the process is what happened. I've seen this before—heck, I caused it during testing. That was the easy part.Yeah there must have been a formatting issue. My bet is the old Fusion drive had 28 GB of space, and when it was migrated a 28 GB partition was created to house that drive's data.
All depends on how you migrated it. There are so many ways to do it, all of which have slightly different results. You might be able to get rid of the partition by reclaiming the space onto the parent drive. It's been a while since I've handled that, but I seem to recall that if you boot the Mac in recovery mode to access Disk Utility at the boot level, it can merge disks.