Trouble mounting High Sierra start up disk from OS X 10.9
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).