9 Copying CD-ROMs and DVDs

Ddrescue may be better than dd for copying recordable CD-ROMs because the two lead out sectors at the end of some of them may cause a read error that prevents the whole last record from being copied by dd, potentially losing data. Also dd may create an image larger than the original if the ‘sync’ conversion and a block size larger than the sector size are specified.

In the special case of reading CD-ROMs (but not DVDs), the specialized tool dvdisaster may be a better option than ddrescue for recovering data because dvdisaster can read and analyze raw CD sectors, which ddrescue can’t.

Recordable CD and DVD media keep their data only for a finite time (typically for some years). After that time, data loss develops slowly with read errors growing from the outer region towards the inside. It is a good idea to make two (or more) copies of every important CD-ROM/DVD you burn so that you can later recover them with ddrescue.

If you have only one copy of a CD-ROM or DVD that fails when being copied, and if you have access to multiple optical media drives, you have a better chance of recovering the bad sectors since one drive may fail to read a particular sector, but another drive might be able to squeeze the data out of it, depending on the laser frequency and the sensitivity of the laser-sensor that reads the reflected laser light.

Example 1: Rescue a CD-ROM in /dev/cdrom.

ddrescue -n -b2048 /dev/cdrom cdimage mapfile
ddrescue -d -r1 -b2048 /dev/cdrom cdimage mapfile
  (if bad-sector size is zero, cdimage now contains a complete image
   of the CD-ROM and you can write it to a blank CD-ROM)

Example 2: Rescue a CD-ROM in /dev/cdrom from two copies.

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

Example 3: Rescue a CD-ROM in /dev/cdrom using two CD drives from two different computers, writing the image into an USB drive mounted on /mnt/mem.

ddrescue -n -b2048 /dev/cdrom /mnt/mem/cdimage /mnt/mem/mapfile
ddrescue -d -r1 -b2048 /dev/cdrom /mnt/mem/cdimage /mnt/mem/mapfile
  (umount the USB drive and move both USB drive and CD-ROM to second
   computer)
ddrescue -d -r1 -b2048 /dev/cdrom /mnt/mem/cdimage /mnt/mem/mapfile
  (if bad-sector size is zero, /mnt/mem/cdimage now contains a complete
   image of the CD-ROM and you can write it to a blank CD-ROM)

Example 4: Merge the partially recovered images of 3 identical DVDs using their mapfiles as domain mapfiles.

ddrescue -m mapfile1 dvdimage1 dvdimage mapfile
ddrescue -m mapfile2 dvdimage2 dvdimage mapfile
ddrescue -m mapfile3 dvdimage3 dvdimage mapfile
  (if bad-sector size is zero, dvdimage now contains a complete image
   of the DVD and you can write it to a blank DVD)

Example 5: 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 -d -r1 -b2048 /dev/cdrom cdimage1 mapfile1
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 -d -r1 -b2048 /dev/cdrom cdimage2 mapfile2
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
  Input files merged successfully.
lziprecover -tv backup.tar.lz
  backup.tar.lz: ok