Cloning an entire disk is a very useful thing to do, to be able to generate a complete, exact copy of every bit of data stored on it at a particular point of time comes in very handy in several ways. It can be a tricky task to accomplish as it needs to be perfect to be useable but fortunately these days there are a growing number of excellent disk cloning tools, both free/opensource and commercial, which make the process of cloning disks relatively simple.
Although the term “cloning” is often used there are in fact a couple of variations and actually a “clone” is an exact copy of every bit so that it is identical to the original drive and can just be swapped with it allowing the system to work as normal as if nothing changed (which is true since it is an identical copy). An alternative type of cloning is called an “image” which is a snapshot of all the data on a disk which is all compressed into a single file and which can then be uncompressed onto a new location whenever required.
Once you have a clone you can then use it as a full backup to restore a system from or you can use it for rapidly provisioning new computers or servers which is extremely useful. Also a clone makes it very easy to upgrade a system or transfer to a new one and be up and running instantly with everything in place exactly as it was on the original.
Probably the first widely released tool for cloning was Ghost which was developed in 1995 and runs on Windows. It quickly became popular and was soon acquired by Symantec. It was however discontinued in 2013.
One of my current favorites is Clonezilla which is very popular these days and being free no doubt contributes to that popularity. It runs on Linux but can clone pretty much any type of filesystem including those used on Windows, Macs, Unix and of course Linux. Unlike Ghost it is still around and going strong. Particularly useful is its ability to run from a USB stick making it great for techs on the road to use it for full system backup and restores.
Macrium Reflect is a Windows based cloning application which comes in both free and commercial versions. Naturally the free version is somewhat limited compared to the much better featured commercial version but even the free one is a useful tool for home users, supporting the usual imaging and cloning functions.
Acronis True Image is a Windows based backup and cloning application which supports all the usual Windows filesystems but also Linux formats such as ext2/3/4 and others. It is able to backup a disk, even the main system disk on which the OS is running, while it is in use which is certainly a handy feature.
dd – it doesn’t get much simpler than dd which is a Unix/Linux command line tool. Cloning a disk is just a matter of running a command like this: dd if=/dev/sda of=/dev/sdb bs=32M and you can basically just reverse the order to do a restore. It can also pipe the data straight to gzip to produce a compressed image. Being Unix based means it also runs on Mac OS/X systems as well. Of course one has to be careful not to get the order of the params mixed up or you could end up with a bad day. It is also important to ensure the partition being cloned is not being used at the time since dd runs at the block level and knows nothing of filesystems which could easily lead to corrupt data on the clone.
Hopefully you found this short list of cloning tools informative but for a full list of disk cloning software check out this page on Wikipedia.