OS X: Imaging and repairing a volume that won't mount #

The HFS+ volume on the following USB hard drive would not mount in Finder:

   $ diskutil list
   ...
   /dev/disk3
   #:                    TYPE NAME       SIZE       IDENTIFIER
   0:  FDisk_partition_scheme           *1.0 TB     disk3
   1:              DOS_FAT_32 scratch    500.1 GB   disk3s1
   2:               Apple_HFS foobar     500.1 GB   disk3s2

so mounting in Terminal was attempted:

   $ sudo mount -t hfs /dev/disk3s2 ~/temp/
   Password:
   mount_hfs: Permission denied

again:

   $ sudo /System/Library/Filesystems/hfs.fs/hfs.util -MU disk3s2 ~/temp/ fixed readonly nosuid nodev
   Password:
   mount_hfs: Invalid argument

and again:

   $ sudo diskutil mount readOnly /dev/disk3s2
   Password:
   Volume on disk3s2 failed to mount
   If the volume is damaged, try the "readOnly" option

to no avail.

Next, the HFS+ volume was imaged with ddrescue:

   $ sudo ddrescue -v /dev/disk3s2 ~/temp/foobar.dmg ~/temp/logfile.log

Attempting to mount the image failed as well:

   $ hdiutil attach -nomount ~/temp/foobar.dmg
   /dev/disk5

   $ diskutil list
   ...
   /dev/disk5
   #:   TYPE NAME     SIZE       IDENTIFIER
   0:        foobar   *500.1 GB   disk5

   $ mount -t hfs -r /dev/disk5 ~/temp/
   mount_hfs: Invalid argument

   $ mount -t hfs -r /dev/disk5s0 ~/temp/
   GetMasterBlock: Error 2 opening /dev/rdisk5s0
   mount_hfs: No such file or directory

   $ mount /dev/disk5 ~/temp/
   mount: You must specify a filesystem type with -t.

   $ diskutil mount readOnly /dev/disk5
   Volume on disk5 failed to mount; if it has a partitioning scheme, use "diskutil mountDisk"
   If the volume is damaged, try the "readOnly" option

   $ diskutil mountDisk readOnly /dev/disk5
   Volume(s) mounted successfully

(Despite the promising "Volume(s) mounted successfully" message, nothing was mounted.)

   $ mount -t hfs -r /dev/disk5s0 ~/temp/
   GetMasterBlock: Error 2 opening /dev/rdisk5s0
   mount_hfs: No such file or directory

   $ mount -t hfs -r /dev/disk5 ~/temp/
   GetMasterBlock: Error 16 opening /dev/rdisk5
   mount_hfs: Resource busy

Happily, a blog post by bbum entitled Recovering Disk Images with DiskWarrior held the answer:

sudo hdiutil mount -nomount -readwrite ~/temp/foobar.dmg
The image should now be attached and appear in DiskWarrior / Disk Utility for repair.

Indeed, DiskWarrior repaired the image successfully:

DiskWarrior has successfully built a new optimized directory for the disk named "foobar." The new directory is ready to replace the original directory.
...
The original directory is damaged and it was necessary to scavenge the directory to find file and folder data.
...
Comparison of the original and replacement directories could not be performed because the original directory was too severely damaged. It is recommended that you preview the replacement directory.

All errors in the directory structure such as tree depth, header node, map nodes, node size, node counts, node links, indexes and more have been repaired.
18 Files had a directory entry with an incorrect text encoding value that was repaired.
1 Folder had an incorrect item count that was repaired.
43 Folders had a directory entry with an incorrect custom icon flag that was repaired.
2 Folders had a directory entry with an incorrect text encoding value that was repaired.
Incorrect values in the Volume Information were repaired.

which was then mountable in Finder with just a double click.

UPDATE: On a related note, Recovering a non-readable disk on Mac OS X describes resolving a "The disk you inserted you inserted was not readable by this computer" error caused by a corrupt partition map via sudo gpt recover /dev/diskx or diskutil repairDisk /dev/diskx

***

In another recent instance, an 8GB USB flash drive would not mount or even be recognized by the OS. After a dozen or so attempts, Disk Utility finally registered its presence, but reported that it was unformatted. diskutil list revealed it had been assigned to /dev/disk2, so imaging was begun:

sudo ddrescue -v /dev/disk2 ~/temp/flashdrive.dmg ~/temp/logfile.log

Over 12 hours later, imaging was complete.

As the image would not mount, testdisk was used to repair the partition table:

sudo testdisk ~/temp/flashdrive.dmg

(The testdisk steps are very simple, but here's a text-based screen recording of the process made with the awesome TermRecord, a "Terminal session recorder that outputs self-contained HTML".)

When testdisk completed, double clicking flashdrive.dmg mounted it easily in Finder.

/mac | Sep 10, 2015

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