Reclaim unaccessable harddrive space on mojave
Just faced with the same problem. I have Yosemity, separated drive without any OS and Mojave on one SSD 250. And I cant add this free space to my Mojave. only to Yusemity. If I'm not mistaken the problem is that your OS is situated on the end of the disk. I dont know how to describe it correctly, so imagine it takes not 0-50% of SSD, but 51-100%. 
IMHO its bullshit, but OSx developers sometimes doing things stupid AF.
Sorry for offtop, so you need to move this drive to the beginning of your SSD and then you can enlarge its space.
Honestly, I haven't fount the sollution yet, and I suppose I will not even search, cuz' its friday's evening=) But I suppose you'll manage that)
I hope it will help you, good luck!