Creating Disk Images Revision as of Wednesday, 26 June 2024 at 10:57 UTC

Floppy/CD/DVD drives

Creating ISOs

The venerable dd is fantastic for this. Let’s say your device is /dev/cdrom:

dd if=/dev/cdrom of=new_cdrom.iso

This will create an ISO9660-compliant image.

Mounting ISOs

ISO files can be loop
mounted like so:

mount -o loop new_cdrom.iso /mount/point

A random bunch of files/directories

For this example, I want to create an ISO9660-compliant image of a
directory, /home/nikhil/Documents:

mkisofs -o /home/nikhil/Documents_backup.iso /home/nikhil/Documents

Important: mkisofs can only create ISOs from directory trees that
are a maximum of 6 levels deep! Any more and it will croak.

Hard Drives

This is quite tricky. You might want to consider
Mondo for its awesomeness. While it
does create ISOs, their contents are compressed and divvied up into
many separate files. If you want an ‘image’ that you can carry around
and mount, read on.

Using dd

Let’s say your HDD is labeled /dev/sdb. You can use dd to create a
block-by-block replica image of this drive like so:

dd if=/dev/sdb of=hard_drive_backup.img conv=sync,noerror

Simple, eh? What’s tricky is that dd will go through all blocks,
even the unallocated ones. This means that if you have a 300GB drive
with just 10GB of data, you’re going to get an image that’s 300GB in
size (can you see why Mondo or Ghost make more sense?)

The conv=sync,noerror will tell dd to write ‘something’ to the image
in case it encounters a bad block.

Speeding up the process

We could use a bigger block size. Using the default 512-byte block size
can slow things down considerably.

dd if=/dev/sdb of=hard_drive_backup.img conv=sync,noerror bs=64K

Making the process space-efficient

We could also use good ‘ol gzip or bzip2 like so:

dd if=/dev/sdb conv=sync,noerror bs=64K | gzip -c > hard_drive_backup.img.gz

Mounting a dd image

The resultant image from the previous steps is absolutely not
ISO9660-compliant. Any attempts to mount it as such might result in mild
embarrassment. To use the image, try fdisk:

[user@localhost mondo_archives]# fdisk -ul hard_drive_backup.img  
last_lba(): I don't know how to handle files with mode 81a4  
You must set cylinders.  
You can do this from the extra functions menu.  
  
Disk hard_drive_backup.img: 0 MB, 0 bytes  
255 heads, 63 sectors/track, 0 cylinders, total 0 sectors  
Units = sectors of 1 * 512 = 512 bytes  
   
            Device Boot           Start        End   Blocks   Id  System  
hard_drive_backup.disk1             63      208844   104391   83  Linux  
hard_drive_backup.disk2          208845    2313359  1052257+  82  Linux swap / Solaris  
hard_drive_backup.disk3         2313360   78156224  37921432+ 83  Linux  
Partition 3 has different physical/logical endings:  
    phys=(1023, 254, 63) logical=(4864, 254, 63)

The tricky part

To mount the third partition (you know, the one with all the data on
it), you need some quick arithmetic: Multiply the starting cylinder by
the block size and use it as the offset for the mount command.

For example: In the output above, the offset would be
512 x 2313360 = 1184440320. You would then use this value like so:

mount -o loop,offset=1184440320 hard_drive_backup.img /mount/point