I have a Mac mini late 2012/2.5 GHz Intel Core i5, with 2 disks, one SSD, and a HDD. The SSD has the most recent High Sierra and the second drive OS X 10.9.5.
After many attempts to force High Sierra to connect my Bluetooth keyboard, I decided to set Maverick as the boot drive and restart to check whether the keyboard is detected there.
In fact, it was, but now it does not mount the SSD with High Sierra and even in the terminal mode I get the message "unknown special file or file system."
Any ideas how to force it to mount again, besides upgrading the 10.9 to Sierra?