Disk won't mount Microsoft Basic Data 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
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.