Information and Recommendations on Cloning Hard Drives

Cloning Benefits

Cloning a hard drive is when you create an exact copy of one drive's contents to another, which has been made quite simple with the availability of high-capacity external drives, simple connectivity, and highly developed cloning software. Many troubleshooting guides include recommendations for "cloning" your system's boot drive. Various MacFixIt articles references this, and it is advocated in discussion forums and other places as an alternate means of backing up the hard drive before performing system maintenance, system alterations, or software upgrades.

Unlike conventional backup systems such as Time Machine, which keep multiple versions of files and track changes over time, cloning only makes one copy of the files on a disk; however, it does have one major advantage in that the clone is bootable. This means that in the event of a hardware failure or major software corruption you can immediately boot off the cloned drive and be running again in a matter of seconds.

Beyond bootability, cloning also can make your work highly portable. For instance, if you have Macintosh computers available in several locations, instead of transferring files back and forth between them, you can clone your drive to a portable external disk and then boot off of it from every machine you work on. This will ensure you have the proper programs installed, and you won't have to worry about strange settings that might interfere with your work flow. You take everything with you, without having to lug around a laptop.

In addition to the computing benefits from cloning, its popularity has arisen partly from the simplicity involved in cloning the contents of one drive to another. All you need is an external drive and cloning software, some of which is included in OS X.

The hardware setup

In order to prepare for cloning your drive, you will first need to have a free drive partition to clone to. This partition can be on any locally mountable drive (internal or external), and can be any size as long as it is at least large enough to accommodate the size of the files on the source disk. Despite the option for this minimum size, we recommend making the clone as large as possible, and preferably the same size as the internal boot disk, which will accommodate clone growth over time.

To find the minimum space required for the clone partition, select your boot drive and get information on it. The number that is displayed next to "Used" in the "General" section is the minimum number of GB that will be needed for the cloned partition. A general rule of thumb is in order to run the system without major slow downs: you will need at least the amount of space to hold everything currently on your hard disk, plus an additional 10 percent or so for virtual memory space. For instance, a drive with 61GB of used space would need at least 68GB free on the clone (61GB 6.1GB free, rounded up).

Even though cloning can be done using minimum drive space, we recommend to plan ahead for drive growth, especially if you plan on booting from and working with files on the clone at some point. If possible, making the clone the same size as the boot drive is the preferred practice.

The software setup

Beyond the hardware and drive size considerations, you will need to use some cloning software to create the clone. While there are well-known third-party packages for creating clones, many people overlook the cloning utilities that come with OS X. In OS X, the "Disk Utility" application can restore one volume to another one, and create a working copy of the system.

To clone with Disk Utility, select a local disk drive in the device list and click the "Restore" tab. Then drag the boot drive to the "Source" box, and your destination partition to the "Destination" box, and click the "Restore" button. You will have the option to erase the destination partition before restoring, which you should do unless the volume is already empty.

This process is convenient, however, unlike third-party solutions there are no ways to schedule Disk Utility to create these clones. Regardless, you can schedule the underlying "asr" (Apple Software Restore) terminal command that Disk Utility utilizes, which can be done using Automator, Applescript, or other scripting solutions that can send commands to the Terminal. To use the "asr" command, consider the following example:

In this example, the command will restore the mounted source volume to the mounted target volume, using the disks at the given mount paths for "Source" and "Clone." For a standard boot drive, the full "Source" path would be /Volumes/"Macintosh HD," and the destination would be the name of the destination drive. For more information on the "asr" command, read the manual page by entering "man asr" in the Terminal or visit the following Web site: http://developer.apple.com/DOCUMENTATION/DARWIN/Reference/ManPages/man8/asr.8.html

Other than the built-in options, there are a couple of robust cloning software packages that people have preferred for various reasons, which include support for various cloning methods as well as options for scheduling. In addition, their cloning routines are faster and in some instances more successful than Disk Utility.

NOTE: Carbon Copy Cloner was recently updated, so be sure you have the latest version if you are planning on using that software

Cloning methods

When you clone a drive, you are either doing file-level cloning or block-level cloning. Each of these methods have unique benefits, though not every cloning package supports them, such as in Disk Utility where file-level cloning is the only option.

File-level cloning is where the system will copy files as they are organized on the disk, but will place them in any available spot on the clone disk. This usually means the files get written to one section of the drive, which can be taken advantage of if you want to defragment your drive for various reasons.

Unlike file-level cloning, which alters the physical location of files on the disk, block-level cloning is like a virtual photocopy of the drive structure and on a per-block basis copies the layout of the source drive to the destination drive. This keeps the files as they are on the disk, and ensures they do not get moved to different locations on the cloned drive. This will not make a difference to most people, but in some instances block-level cloning may be preferred.

Testing and troubleshooting

Once the clone has been made, you will need to test it out and ensure everything went smoothly. The easiest way to do this is to simply boot off of it. With the drive connected to the system, reboot and hold the Options key to bring up the boot menu. Then select the cloned drive to boot from, and if everything worked out you should see your desktop and files as if you were booted to the internal drive.

It may also benefit you to run disk verification and permissions checks on the drive using Disk Utility or other drive maintenance software to ensure the drive structure is intact. This can be done either booted off the drive or done more thoroughly when running from another boot drive.

Basic use and recommendations

Once the drive has been cloned, you can boot from it using other computers or allow it to be a backup for testing new software or system configurations. If you need to restore your internal boot drive from the clone, the steps are exactly the same as creating the clone initially, only that you're using the internal drive as the destination drive. Unlike Time Machine, which requires you to boot to your Leopard DVD and restore using various utilities, you can immediately boot to the clone and either work from it or restore at a time of your choosing. Regardless of the files you create on the clone drive, when you restore it to the internal drive all new files will be copied over.

There are several things to keep in mind with clones. For one, clones do not need to be on their own drive, and you can utilize both the cloning and Time Machine on separate partitions of the same drive. There are many combinations of drives that can be used, and its worth exploring the options that will best suit your setup. Additionally, even though some people may argue either way on this, they're not a replacement for conventional backup systems since they don't keep a history of file changes. Time Machine's "snapshots" are exceptionally useful for finding preferred or lost versions of files, and therefore one setup you might consider is using clones in conjunction with Time Machine and other backup systems. Using Time Machine will allow for hourly backups, and scheduling clones to be made once a day can be easily set up to ensure you can immediately boot in the event of a major software or hardware fault and access your most recent files. Doing this will ensure you are able to keep working with minimum interruption to your workflow.

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