is a category of software which copies the contents of one computer
hard disk to another or into an "image" (a file) (Sometimes referred to
as Ghost Imaging). Often, the contents of the first disk are written to
an image file as an intermediate step, and the second disk is loaded
with the contents of the image. This procedure is also useful when
moving to a larger capacity disk.
There are a number of notable uses for disk cloning software. These include:
- Reboot and restore - a technique in which the disk of a
computer is automatically wiped and restored from a "clean", master
image, which should be in full working order and should have been swept
for viruses. This is used by some cybercafes and some training and
educational institutes, and helps ensure that even if a user does
misconfigure something, downloads inappropriate content or programs, or
infects a computer with a virus, the computer will be restored to a
clean, working state. The reboot and restore process can either take
place irregularly when a computer shows signs of malfunctioning, on a
regular basis (e.g., nightly) or even, in some cases, every time a user
logs off, which is the safest approach (although that does involve some
- Provisioning new computers - Provisioning with a
standard set of software so that a new user is ready to go straight
away with a complete application suite and does not have to waste time
installing individual applications. This is often done by original
equipment manufacturers and larger companies.
- Hard drive upgrade - An individual user may use disk copying (cloning) to upgrade to a new, sometimes larger, hard disk.
- Full system backup - A user may create a comprehensive backup of their operating system and installed software.
- System recovery - An OEM can provide media that can restore a computer to its original factory software configuration
- Transfer to another user - A system sold or given to
another person may be reset by reloading a known, previously-saved
image that contains no personal files or information.
How it works
This article is specific to disk cloning on the x86 platform;
specific details may not apply to other platforms.
To provision the hard disk of a computer without using disk cloning
software, the following steps are generally required for each computer:
- Create one or more partitions on the disk
- Format each partition to create a file system on it
- Install the operating system
- Install device drivers for the particular hardware
- Install application software
With disk cloning, this is simplified to:
- Install the first computer, as above.
- Create an image of the hard disk (optional)
- Clone the first disk, or its image, to the remaining computers
Before Windows 95, some computer manufacturers used hardware disk
copying machines to copy software. This had the disadvantages of
copying not just the used data on the disk, but also unused sectors, as
the hardware used was not aware of the structures on the disks. A
larger hard disk could not be copied to a smaller one, and copying a
smaller one to a larger left the remaining space on the new disk
unused. The two disks required identical geometries.
Other manufacturers and companies partitioned and formatted disks
manually, then used file copy utilities or archiving utilities, such as
tar or zip to copy files. It is not sufficient simply to copy all files
from one disk to another, because there are special boot files or boot
tracks which must be specifically placed for an operating system to
run, so additional manual steps were required.
Windows 95 compounded the problems because it was larger than earlier
popular operating systems, and thus took more time to install. The long
filenames added to the FAT filesystem by Microsoft in Windows 95 were
not supported by most copy programs, and the introduction of the FAT32
filesystem in 1997 caused problems for others. The growth of the
personal computer market at this time also made a more efficient
Ghost was introduced in 1996 by Binary Research. It initially supported
only FAT filesystems directly, but it could copy but not resize other
filesystems by performing a sector copy on them. Ghost added support
for the NTFS filesystem later that year, and also provided a program to
change the Security Identifier (SID) which made Windows NT systems
distinguishable from each other. Support for the ext2 filesystem was
added in 1999.
Competitors to Ghost soon arose, and a features war has carried on to
the present day. Many disk cloning programs now offer features which go
beyond simple disk cloning, such as asset management and user settings
On UNIX based computer systems, dd was more commonplace due to the lack
of filesystem support in Ghost.
Two machines with identical names are said not to be allowed on the
same network, and, for Windows NT and its successors, two machines with
identical security IDs (SIDs, aka Security Identifier) are said not to
be allowed on the same Active Directory domain.
A disk cloning program should change these as part of copying the disk
or restoring the image. Some operating systems are also not well suited
to changes in hardware, so that a clone of Windows XP for example may
object to being booted on a machine with a different motherboard,
graphics card and network card, especially if non-generic drivers are
used. Microsoft's solution to this is Sysprep, a utility which runs
hardware detection scans and sets the SID and computer name freshly
when the machine boots. Microsoft recommends that Sysprep be set up on
all machines before cloning, rather than allow third party programs to
configure them. Similarly, Linux systems simply require the necessary
kernel modules to be available (or compiled directly into the kernel),
in order to support new hardware when the machine boots. However there
are ways to help make images for cloning with Windows more portable.
One such example would be a product called Universal Imaging Utility
from Binary Research (original developers of Symantec's Ghost) which
incorporates a large number of hardware device drivers into the sysprep
Actually, the problem with duplicated SIDs in a Workgroup of computers
running Windows NT/2K/XP is only related to different user accounts
having the same SID. This could lead to unexpected access to shared
files or files stored on a removable storage: If some ACLs (Access
control lists) are set on a file, the actual permissions can be
associated with a user SID. If this user SID is duplicated on a cloned
computer (because the computer SID is duplicated and because the user
SIDs are built based on the computer SID + a sequential number), a user
of a second computer (cloned from the first one) could have access to
the files that the user of a first computer has protected.
When it comes to "Domain SID", the Domain SID is recomputed each time a
computer enters a domain. Thus, all the "post-cloning operations" that
are based on "leave the domain and then rejoin the domain" will
actually cause a re-creation of the Domain SID for the computer that
joins the domain.
In other words, duplicated SIDs are usually not a problem with
Microsoft Windows systems
There are files in some Microsoft operating systems (called BOOTSECT.*)
which are copies of the Boot Partition Block (BPB) used by alternate
operating systems that Microsoft Windows loader (NTLDR) can load.
BOOTSECT.* files may have to be altered if partition sizes or layouts
are changed during the clone.
Linux systems usually boot using either the LILO or GRUB bootloaders.
These contain lists of absolute disk sectors in their MBR, which must
be altered by the cloning program as the files they refer to are likely
not to be in the same location of the destination disk. For example, if
the original boot loader script points to the system being on a disk on
channel 0 and the system being of the second partition, the target
computer will need to have the same configuration.
Another useful (but not perfect) tool is NewSid
A disk cloning program needs to be able to read even protected
operating system files on the source disk, and must guarantee that the
system is in a consistent state at the time of reading. It must also
overwrite any operating system already present on the destination disk.
To simplify these tasks, most disk cloning programs can run under an
operating system different from the native operating system of the host
computer, for example, MS-DOS or an equivalent such as PC-DOS or
DR-DOS, or Linux. The computer is booted from this operating system,
the cloning program is loaded and copies the Windows file system. Many
programs (e.g. Acronis True Image) can clone a disk, or make an image,
from within Windows, with special provision for copying open files; but
an image cannot be restored onto the Windows System Drive under
A disc cloning program running under non-Windows operating systems must
have device drivers or equivalent for all devices used. The
manufacturers of some devices do not provide suitable drivers, so the
manufacturers of disk cloning software must write their own drivers, or
include device access functionality in some other way. This applies to
tape drives, CD and DVD readers and writers, and USB and FireWire
drives. Cloning software contains its own TCP/IP stack for multicast
transfer of data where required.
The simplest method of cloning a disk is to have both the source and
destination disks present in the same machine, but this is too
restrictive. Disk cloning programs can link two computers by a parallel
cable, or save and load images to a network drive. As disk images tend
to be very large (usually a minimum of several hundred MB), performing
several clones at a time puts excessive stress on a network. The
solution is to use multicast technology. This allows a single image to
be sent simultaneously to many machines without putting greater stress
on the network than sending an image to a single machine.
Although disk cloning programs are not primarily backup programs,
they are sometimes used as such. A key feature of a backup program is
to allow the retrieval of individual files without needing to restore
the entire backup. Disk cloning programs either provide a Windows
Explorer-like program to browse image files and extract individual
files from them, or allow an image file to be mounted as a read-only
filesystem within Windows Explorer.
Some such programs allow deletion of files from images, and addition of
- List of disk cloning software
- Comparison of disk cloning software
- Disk wiping
- Disk mirroring
- Disk image
- dd (Unix)
- Live USB
- Recovery disc
- Security Identifier