Ok, I think this is really important.
I formatted an external drive I have using GUID Mac OS Journaled. Plugged it into the Yosemite and tried to use EFI Mounter v3, but it gave me the same error it does with the internal SSD.
Then I connected it back to my mac mini and run EFI Mounter v3 from there and it mounts just fine!
What do you make of it?