My Mac's HDD is not opening. When I enter diskutil umountDisk dev/disk0 shows:

At least one volume could not be unmounted

sais-MacBook-Pro:~ sk$ diskutil list
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *121.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2: FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF               101.1 GB   disk0s2
   3:                  Apple_HFS Recovery HD             19.4 GB    disk0s3
   4:                 Apple_Boot Recovery HD             650.0 MB   disk0s4
  • @klanomath - you're clearly the resident guru on this; I'd flag a mod or use the chat to get these pushed into the correct order as to which should be master/similar/dupe etc – Tetsujin Jan 16 '17 at 19:31
  • @Tetsujin I have partly rewritten my answer to "Recover data from partition showing as FFFF" and added a link to "How do I recover/fix my OS X partition?" in case hexdump isn't available. – klanomath Jan 16 '17 at 19:35
  • @klanomath - nice work, yet again. – Tetsujin Jan 16 '17 at 19:36

Try to boot from Apple_boot Recovery HD

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