Hard drive disappears when external drive is used to boot

I’m using a San Disk SSD as a backup drive (Super Duper) for my Mid 2012 MacBook Pro. When I use that drive to boot a different (slightly older) Macbook Pro, it boots just fine, but the MBPs internal drive does not mount. I can see it in Disk Utility, but the mount option is greyed out.

Any idea as to what is happening? The older MBP has High Sierra on its usual boot drive. The backup disk has El Capitan - don’t know if that would make a difference.

I ran first aid on the internal disk and it came back with nothing. And if I restart without the external drive, the machine boots fine on the internal disk.

High Sierra started using the APFS file system for boot drives (I believe specifically for SSD boot drives), and El Capitan cannot mount APFS drives. That may be the reason.

Hmmmmmmm, interesting. My goal is to restore the High Sierra machine using the clone on the El Capitan drive. Any idea how I can accomplish that in this scenario?

I’m confused about what you mean with “restore the High Sierra machine using the clone on the El Capitan drive”. (It’s also a little puzzling that you have the newer OS on the older Mac and vice-versa, but no matter.) What do you want where: on the older Mac, just the files from the newer Mac, but staying with High Sierra; or everything (= files and El Cap)?

The things you might need to use: Target Disk Mode, Migration Assistant, the installer for HS and/or El Cap, your SanDisk clone. And I’m not sure how the OS extended/APFS difference might complicate your task, but it might.

Not sure I’m understanding all the intricacies of the situation, but Migration Assistant will happily migrate a drive running an older OS to a drive formatted as APFS. So restoring a High Sierra machine from the clone of your El Capitan machine should present no issues (barring some of the strange buggy behavior I too often see with Migration Assistant these days).