I had this external 3TB drive split into 2 partitions (1.5 TB each) both Mac OS Extended (journaled). One partition completely disappeared. Disk utility says it is not mounted. I can't mount it, repair it or erase it. The other partition is still there and the files are accessible. The other partition shows up and sometimes I can see the files and sometimes I can't. I tried to run first aid with disk utility, but it says First Aid count not unmount the drive. What do I do?