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.