7 A small tutorial with examples

This tutorial is for those already able to use the dd command. If you don’t know what dd is, better search the net for some introductory material about dd and GNU ddrescue first.

A failing drive tends to develop more and more errors as time passes. Because of this, you should rescue the data from a drive as soon as you notice the first error. Be diligent because every time a physically damaged drive powers up and is able to output some data, it may be the very last time that it ever will.

You should make a copy of the failing drive with ddrescue, and then try to repair the copy. If your data is really important, use the first copy as a master for a second copy, and try to repair the second copy. If something goes wrong, you have the master intact to try again.

If you are trying to rescue a whole partition, first repair the copy with e2fsck or some other tool appropriate for the type of partition you are trying to rescue, then mount the repaired copy somewhere and try to recover the files in it.

If the drive is so damaged that the file system in the rescued partition can’t be repaired or mounted, you will have to browse the rescued data with an hex editor and extract the desired parts by hand or use a file recovery tool like photorec.

If the partition table is damaged, you may try to rescue the whole disc, then try to repair the partition table and the partitions on the copy.

If the damaged drive is not listed in /dev, then you cannot rescue it. At least not with ddrescue.

Example 1: Rescue a whole disc with two ext2 partitions in /dev/hda to /dev/hdb.
Note: you do not need to partition /dev/hdb beforehand, but if the partition table on /dev/hda is damaged, you’ll need to recreate it somehow on /dev/hdb.

ddrescue -f -n /dev/hda /dev/hdb logfile
ddrescue -d -f -r3 /dev/hda /dev/hdb logfile
fdisk /dev/hdb
e2fsck -v -f /dev/hdb1
e2fsck -v -f /dev/hdb2

Example 2: Rescue an ext2 partition in /dev/hda2 to /dev/hdb2.
Note: you need to create the hdb2 partition with fdisk first. hdb2 should be of appropriate type and size.

ddrescue -f -n /dev/hda2 /dev/hdb2 logfile
ddrescue -d -f -r3 /dev/hda2 /dev/hdb2 logfile
e2fsck -v -f /dev/hdb2
mount -t ext2 -o ro /dev/hdb2 /mnt
  (read rescued files from /mnt)

Example 3: Rescue a CD-ROM in /dev/cdrom.

ddrescue -n -b2048 /dev/cdrom cdimage logfile
ddrescue -d -b2048 /dev/cdrom cdimage logfile
  (if errsize is zero, cdimage now contains a complete image of the
   CD-ROM and you can write it to a blank CD-ROM)

Example 4: Rescue a CD-ROM in /dev/cdrom from two copies.

ddrescue -n -b2048 /dev/cdrom cdimage logfile
ddrescue -d -b2048 /dev/cdrom cdimage logfile
  (insert second copy in the CD drive)
ddrescue -d -r1 -b2048 /dev/cdrom cdimage logfile
  (if errsize is zero, cdimage now contains a complete image of the
   CD-ROM and you can write it to a blank CD-ROM)

Example 5: Merge the partially recovered images of 3 identical DVDs using their logfiles as domain logfiles.

ddrescue -m logfile1 dvdimage1 dvdimage logfile
ddrescue -m logfile2 dvdimage2 dvdimage logfile
ddrescue -m logfile3 dvdimage3 dvdimage logfile
  (if errsize is zero, dvdimage now contains a complete image of the DVD
   and you can write it to a blank DVD)

Example 6: Rescue a lzip compressed backup from two copies on CD-ROM with error-checked merging of copies (See the lziprecover manual for details about lziprecover).

ddrescue -b2048 /dev/cdrom cdimage1 logfile1
mount -t iso9660 -o loop,ro cdimage1 /mnt/cdimage
cp /mnt/cdimage/backup.tar.lz rescued1.tar.lz
umount /mnt/cdimage
  (insert second copy in the CD drive)
ddrescue -b2048 /dev/cdrom cdimage2 logfile2
mount -t iso9660 -o loop,ro cdimage2 /mnt/cdimage
cp /mnt/cdimage/backup.tar.lz rescued2.tar.lz
umount /mnt/cdimage
lziprecover -m -v -o backup.tar.lz rescued1.tar.lz rescued2.tar.lz

Example 7: While rescuing the whole drive /dev/hda to /dev/hdb, /dev/hda freezes up at position 12345678.

ddrescue -f /dev/hda /dev/hdb logfile       <-- /dev/hda freezes here
  (restart /dev/hda or reboot computer)
  (restart copy at a safe distance from the troubled sector)
ddrescue -f -i 12350000 /dev/hda /dev/hdb logfile
  (copy backwards down to the troubled sector)
ddrescue -f -R /dev/hda /dev/hdb logfile

Example 8: While rescuing the whole drive /dev/hda to /dev/hdb, /dev/hdb fails and you have to rescue data to a third drive, /dev/hdc.

ddrescue -f -n /dev/hda /dev/hdb logfile1     <-- /dev/hdb fails here
ddrescue -f -m logfile1 /dev/hdb /dev/hdc logfile2
ddrescue -f -n /dev/hda /dev/hdc logfile2
ddrescue -d -f -r3 /dev/hda /dev/hdc logfile2

Example 9: While rescuing the whole drive /dev/hda to /dev/hdb, /dev/hda stops responding and disappears from /dev.

ddrescue -f -n /dev/hda /dev/hdb logfile      <-- /dev/hda fails here
  (restart /dev/hda or reboot computer as many times as needed)
ddrescue -f -n -A /dev/hda /dev/hdb logfile
ddrescue -d -f -r3 /dev/hda /dev/hdb logfile

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