Since it’s wholly automatic and completely transparent, it’s silly not to make use of the Mac OS Time Machine utility — as long as you’ve got a large enough external drive for it to work with, of course. I’ve been using it with a 320Gb Seagate FreeAgent Pro external USB drive, but this only gave a backup history of about a month with my 320Gb MacBook Pro drive (which has about 90Gb in use).
So, since I also had an old 500Gb Western Digital MyBook drive lying around, I thought I’d use that for backups instead — not only would its greater capacity offer a few more weeks’ backup history, but its FireWire 800 port should make Time Machine a bit quicker to work with, too.
Rather than simply plug in the MyBook and use it as a new Time Machine drive though, I wanted to transfer my old backups and simply add to them. There’s no obvious way to do this, but it can be done easily enough — here’s how.
Before you start, I’ll point out that you’ll need a second drive with a single Mac OS Extended (Journaled) partition that’s at least as large as the original Time Machine partition (it doesn’t matter what you call it). You can set this up in Disk Utility.
Connect both old and new Time Machine drives to your Mac. Then open Time Machine Preferences (you can right-click on the Menu bar icon for this) and turn off Time Machine.
Eject the original Time Machine drive and then reconnect it to have Mac OS recognise it as a standard external drive (or just unmount the partition in Disk Utility and mount it again).
Open Disk Utility and you should see two standard external drives connected — the original Time Machine drive and the new, larger drive. Click the Restore tab, then drag the original Time Machine partition on the left of the dialog box to the Source box on the right — it should then appear in the Source box. Drag the icon for the new Time Machine (Untitled 1 in my screen shot) drive to the Target box in the same way. Tick the box for ‘Erase destination’ — the disk copy will fail with a “Restore Failure: Could not restore – Operation not permitted: error” if you don’t.
Click the Restore button and once you’ve double-checked the configuration (make sure you’ve selected the correct Source and Target drives!), you can proceed. The disk copy will take a few hours and you’ll need to wait around for the new disk’s contents to be verified, too. The process took four or five hours for my 320Gb > 500Gb upgrade — I left it running overnight.
When the disk copy is complete, eject the old Time Machine drive. Open Time Machine Preferences again (see Step 1). Turn on Time Machine, then click the Change Drive button and select your new, larger drive — the drive icon will change from orange to green. Enter Time Machine to check that your backup history has been preserved and once you’re happy that it has, you can erase the old Time Machine drive and use it for something else.