Trouble mounting High Sierra start up disk from OS X 10.9 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?
Your SSD is formatted to an APFS container with several APFS volumes including the High Sierra system volume.

Mavericks and all other OSs (except Sierra with "experimental support" and High Sierra) natively don't contain drivers to read from or write to APFS.
Paragon offers a read-only driver for 10.10-10.12.

Consequently you can't mount APFS in Mavericks.



You can backup your High Sierra volume, reformat the SSD to HFSJ+ or CoreStorage and restore HS there though, but this is not recommended.

So if you want to access files and folders on the High Sierra volume in Mavericks use a third volume (formatted to HFSJ+/CS) as a transitional volume (i.e. copy all required files from HS to this volume to access them from Mavericks).