Maybe you’re running out of disk space. Or maybe you need to replace an old hard drive that’s making a disturbing click-click-whirr noise. But before you can replace your Mac’s boot drive, you need to clone its contents onto a new one.

1. Collect the hardware and software

The key to cloning is to use a drive dock, a device that lets you connect a raw hard disk (such as the kind you’d install inside a computer) to your Mac. You clone your old drive to the raw drive, and then insert that new drive in your Mac. In addition to cloning drives, you can use a dock to make a backup of your drive, and then store that backup offsite.

While many manufacturers offer Mac- compatible docks, we like NewerTech’s Voyager line (www.newertech.com). It comes in two versions: the Voyager S3 (£46) has a USB 2.0/3.0 port and the Voyager Q (£78) adds eSATA, FireWire 400, and FireWire 800 (from www.thebookyard.com). Both accept a 2.5in or 3.5in Serial ATA (SATA) drive.

Choose your original boot drive from the Copy drop-down menu, your new drive from the To menu, and Backup – All Files from the Using menu

You’ll also need a cable to connect the dock to your Mac. Docks come with a variety of connectors; make sure you have the right cable for connecting at the highest speed possible. For the Voyager S3, USB 2.0 is as fast as you can go (unless you want to add USB 3.0 to your Mac via an add-in card). FireWire 800 may be your best bet on most Macs.

For the raw drive itself, you have a huge number of choices. Your starting parameter is a 2.5in SATA drive for a laptop or Mac mini, or a 3.5in SATA drive for all other Macs. Our rule of thumb: look for the sweet spot before which the price per gigabyte (or per terabyte) starts going up. So if a 1TB drive is £40, a 1.5TB drive is £90, and a 2TB drive is £120, we’d argue that the 1TB is the best deal (assuming it meets your storage needs). But if a 1TB drive is £50, a 1.5TB is £70, and a 2TB is £100, the 1.5TB drive might be more sensible.

Finally, you need cloning software. Two programs are standard: Carbon Copy Cloner (www.bombich.com) and SuperDuper (www.shirt-pocket.com). Both have free versions that can clone disks.

2. Make your clone

First, back up your current internal drive. Then connect your dock to a power source and to your Mac, and insert the replacement drive into it. Use Disk Utility (in /Applications/Utilities) to format the drive. When the formatting is complete, click on the Partition tab in that same app. Check to make sure the current partitioning scheme is the right type for your system. Finally, click on the Options button. For an Intel Mac, make sure that GUID Partition Table is selected. For a PowerPC Mac, choose Apple Partition Map. Click OK and then Apply.

Now launch your cloning app. In Carbon Copy Cloner, choose your internal drive in the Source Disk pop-up menu, and the replacement disk in the Target Disk menu. Make sure Backup Everything is selected from the Cloning Options pop-up menu. A green icon should appear in the Target Disk column next to a label reading, This volume will be bootable. Click Clone.

If you’re using SuperDuper, select the internal drive in the Copy menu and the replacement drive in the To menu on the right. Select Backup – All Files from the Using menu. Click Copy Now. 

Select your old boot drive as the source disk, the new one as the target disk and opt to back up everything

3. Test the clone

When the cloning is complete, open System Preferences, click on Startup Disk, and select your replacement drive. Click Restart. If your system boots up correctly, you have a good clone.

Here’s where you might spot the weakness in this process: how can you tell the clone from the original?

The easiest way to do so is to change the original disk’s name before you restart your Mac (add _old to its name in the Finder). Alternatively, give the two disks distinct icons: Find an icon you like and copy it to the Clipboard. Select the drive in the Finder, choose File > Get Info, click on the icon in the floating window, and press Cmd-V. In either case, after the restart, make sure the correct name or icon appears at the very top on the desktop (where a hard-drive icon usually appears).

If your computer didn’t boot correctly from your clone, check the Startup Disk preference pane to see what’s selected. If the cloned drive isn’t selected, try selecting it again and restarting. You can also hold down the Option key at startup to produce a list of bootable volumes. If the replacement drive isn’t among them, run Disk Utility and repair that drive. 

4. Install the clone

Assuming your clone works, shut down your Mac and swap in the replacement drive as your new internal drive. (Find help on www.apple.com or www.iFixit.com). Your Mac should automatically boot from the new internal drive; it may take longer than usual, since your Mac must sort out which drives are available. If you get a flashing question mark, shut down, make sure cables are connected properly, and try the Option-key startup method described in step three. You can also boot from an OS X installation DVD and run Disk Utility to make sure the drive is OK.

You should now have two distinct backups of your new live replacement drive: the one you made before cloning and the one of the original internal drive. Keep those on hand in case something goes wrong with the new drive. That’s most likely to happen within the first few weeks of operation. If you have a failure and you’ve kept cloning or backing up regularly, you can put your original drive back in your Mac, or use the dock to boot it externally.