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I have an external disk that won't mount on my Mac: 

/dev/disk6 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *2.0 TB     disk6
   1:                        EFI EFI                     209.7 MB   disk6s1
   2:                  Apple_HFS                         2.0 TB     disk6s2

When I first got the disk (from a colleague) it would not mount. I saw that /dev/disk6s2 was set to Microsoft Basic Data so I assumed that was the problem. I ran sudo asr adjust --target /dev/disk6s2 --settype "Apple_HFS" to fix that and now (as seen above) disk6s2 is set to Apple_HFS but it still won't mount.

The output of sudo gpt -r show /dev/disk6 is:

      start        size  index  contents
           0           1         PMBR
           1           1         Pri GPT header
           2          32         Pri GPT table
          34           6         
          40      409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
      409640        2008         
      411648  3906617344      2  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
  3907028992         143         
  3907029135          32         Sec GPT table
  3907029167           1         Sec GPT header

I don't see any problems there but I'm getting into areas I'm not fully familiar with. I have no reason to think nor have I noticed any physical problems with the disk itself.

Any suggested steps or diagnoses?

MacPro (late 2013) OSX 10.11.6

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2 Answers 2

Reset to default
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The asr target option --settype "Apple_HFS" modifies the GPT partition type but not the file system. So the partition "content" is 48465300-0000-11AA-AA11-00306543ECAC (which is an indicator for a normal HFS+ partition) but the file system on disk6s2 still is NTFS/ExFAT/FAT32.

If the disk is empty, simply erase it with Disk Utility to whatever you need.

Changing the partition "content" with gpt or another similar tool doesn't alter the file system. Your volume may not mount if content and file system don't match!

BTW asr is not the proper tool to format volumes, create file systems or to change the partition type. This is rather done with newfs_XY, diskutil and gpt.

To determine or examine the file system you would have to open the volume as raw data and check for specific headers or fs structures.

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  • Thansk! Yes, I'm pretty sure the file system is ExFAT, which should have no problem mounting to a Mac. How can you tell the file system is NTFS/ExFAT/FAT32 from the information I provided? Does the Microsoft Basic Data indicate that?
    – Bleakley
    Commented Aug 18, 2017 at 17:21
  • Also, how do I open/mount the volume as raw data?
    – Bleakley
    Commented Aug 18, 2017 at 17:21
  • @Bleakley You have to use a tool like dd + HexEdit (or HexEdit only) or wxHexEditor. Depending on the file system you have to check the first block, the first ~20 blocks or the last blocks. MBD is a partition type while NTFS/ExFAT/FAT32 are all file systems which may reside on MBD partitions. Examples: NTFS Partition booter sector / exFAT > ~page 19
    – klanomath
    Commented Aug 18, 2017 at 17:26
  • but do I need to have it mounted first to use dd or can dd access unmounted volumes that diskutil sees?
    – Bleakley
    Commented Aug 18, 2017 at 17:29
  • @Bleakley First get the disk identifier: diskutil list> assuming it's disk6: diskutil umountDisk /dev/disk6> sudo dd if=/dev/disk6s2 of=/Users/<username>/Desktop/firstsectors.bin count=32 bs=512 will dd the first 32 blocks to a file firstsectors.bin on your desktop.
    – klanomath
    Commented Aug 18, 2017 at 17:37
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You can fix that probem with Disk Utility/Erase/Security Options/Most Secure

Basicaly it's going to start delete everything. by the way, you don't need to wait it for finishing its job

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  • Could you elaborate what delete everything is? That seems bad to throw out everything on a computer or worse write random data to all the drives. Perhaps we're missing some context and you can edit in more details to make your answer stand alone in answering the question at hand.
    – bmike ♦
    Commented Aug 12, 2018 at 18:41

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