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I have a 1TB generic external hard drive containing a single HFS partition. I originally formatted this using Disk Utility and it worked fine. Now, for some reason, it's not auto-mounting when I start up.

Using mount at the command line gives the following error:

$ sudo mount /dev/disk1s2 /Volumes/Test
/dev/disk1s2 on /Volumes/Test: Incorrect super block.

... but if I use the mount_hfs command it works fine, mounts, and is readable.

$ mount_hfs /dev/disk1s2 /Volumes/Test/

fsck gives me an error about a bad super block:

$ fsck /dev/disk1
** /dev/rdisk1 (NO WRITE)
BAD SUPER BLOCK: MAGIC NUMBER WRONG

... but fsck_hfs -fn /dev/disk1s2 doesn't find any problems and reports that the volume appears to be OK.

In Disk Utility, the drive appears to have a single MS-DOS partition with a curious notice about how it appears to be partitioned for Boot Camp:

Screenshot

I have the Boot Camp HFS driver installed in WIndows 7, and that OS sees the drive/partition normally. What's wrong with my disk?

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3 Answers

up vote 5 down vote accepted

It looks like the problem was the partition table.

$ diskutil list
/dev/disk0   # The internal hard disk
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            402.5 GB   disk0s2
   3:       Microsoft Basic Data Boot Camp               97.1 GB    disk0s3
/dev/disk3   # The external USB drive
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk3
   1:                        EFI                         209.7 MB   disk3s1
   2:       Microsoft Basic Data                         999.9 GB   disk3s2
          # ^-- Hey, that's not right!

The filesystem was intact, but the HFS+ partition on the external drive was mistakenly flagged as Microsoft Basic Data instead of Apple_HFS. I used the GPT fdisk utility to change its type back to HFS+, and it immediately appeared on the desktop and worked normally again.

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I am currently experiencing a very similar issue. I ran the following command, the output of which I included. However, this still did not fix the problem for me, however maybe you will have better luck.

> fsck_hfs -rd /dev/disk1s2
** /dev/rdisk1s2
    Using cacheBlockSize=32K cacheTotalBlock=8192 cacheSize=262144K.
   Executing fsck_hfs (version diskdev_cmds-491~1).
    Journal replayed successfully or journal was empty
** Checking Journaled HFS Plus volume.
** Checking extents overflow file.
** Checking catalog file.
** Rebuilding catalog B-tree.
hfs_UNswap_BTNode: invalid node height (1)
** Rechecking volume.
** Checking Journaled HFS Plus volume.
** Checking extents overflow file.
** Checking catalog file.
** Checking multi-linked files.
** Checking catalog hierarchy.
** Checking extended attributes file.
** Checking volume bitmap.
** Checking volume information.
    invalid VHB attributesFile.clumpSize 
   Volume header needs minor repair
(2, 0)
   Verify Status: VIStat = 0x8000, ABTStat = 0x0000 EBTStat = 0x0000
                  CBTStat = 0x0000 CatStat = 0x00000000
** Repairing volume.
** Rechecking volume.
** Checking Journaled HFS Plus volume.
** Checking extents overflow file.
** Checking catalog file.
** Checking multi-linked files.
** Checking catalog hierarchy.
** Checking extended attributes file.
** Checking volume bitmap.
** Checking volume information.
** The volume EXTERNAL was repaired successfully.
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A post at Apple Support forums indicates that DiskWarrior might help you in this scenario.

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DiskWarrior just lists it as "Unknown Disk" and says it cannot rebuild the directory because the file system is unsupported. (It thinks the file system is 'Windows/PC', just like Disk Utility.). –  Brant Bobby Dec 13 '09 at 21:41
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