Snow Leopard makes changing Time Machine disks a snap

For my money, Time Machine is the greatest feature Apple introduced with Mac OS X 10.5 Leopard, and one of the niftiest OS X features ever. Boasting always-on backups of every version of every important item on your Mac, it's the first computer backup system to grant zero-effort peace of mind to every Mac owner with typical backup needs. 

But Time Machine will eat up space on its dedicated external disk drive. Lack of storage itself isn't the problem, really, when 1-terabyte drives are about $100 and getting cheaper all the time. It's just the hassle of moving up to a bigger drive when Time Machine fills the small one. (The same holds true if you don't need a bigger drive yet but have reason to suspect the health of the current one. NEVER trust your data, including backups, to an old disk making funny grinding noises!)

I recently had cause to move up. My 500 GB external drive was getting crowded, with both the Time Machine backup and a growing video library fighting for elbow room, so I bought a new 1 TB drive to take on Time Machine and give it plenty of room to expand. Snow Leopard made the switch wonderfully easy:

Time Machine has filled the drive. Now what?

First, a look at your options when you've got a full Time Machine drive. It turns out that you have several: 

Leave it alone: Just do nothing. Time Machine, quite brilliantly, won't choke and spit up some "disk is full" error message. It'll keep on saving new data – but it has to sacrifice old data to make room. While that's the best you could ask Time Machine to do under the conditions, it's far from ideal: should you find you need to restore that old data, well, sorry, it's now gone from the backup. 

Start over: Get a new drive and start over. Switching to a new drive is dead simple; see this overview. The drawback, as with the above, is potential loss of files. What if there's some file right now, unknown to you, that's lost on your Mac and clinging to life only inside that Time Machine backup? It won't become part of your new disk's Time Machine backup, and will be lost forever unless you stash the old disk away untouched. To be perfectly safe, you'll want to keep all old Time Machine drive(s), which you can search via the Finder should some file turn up missing on your Mac and your current Time Machine drive.

Move the backup: The solution that keeps all your old backups in one place and doesn't risk any loss is to transfer the Time Machine backup to a new, larger disk drive and pick up from there. You can then do what you like with the old Time Machine drive. 

Moving the backup, the old Leopard way

When Time Machine arrived with Leopard, it didn't offer clear instructions on how to move a backup to a new drive. Mac users figured out a procedure involving the "Restore" tab in the Disk Utility application. There's no need to repeat the procedures here, with complete guides online. I recommend this:

Follow the instructions at How To Move OS X Time Machine Backups To A New Disk, with one change: After Step 8, check the box labeled "Erase Target". Not doing so can result in an error – the process runs for a while, complains "Could not restore – operation timed out" – and fails to go further. (The extra step is further discussed here.) 

Then again, some people have experienced trouble even with that option checked. I, and at least one other Snow Leopard user commenting online, immediately received the error message "Could not validate source – Bad file descriptor" upon clicking "Restore". It's nice that the error popped up with no waiting, but it left me stuck all the same. 

I'm unable to say whether that latter error is peculiar to those trying the above method using Snow Leopard, or whether some Leopard users experience it too. Either way, all my attempts to move the backup to a new disk met with one of the above two errors. 

Fortunately, as a Snow Leopard user I have a different – and completely trouble-free – solution:

Moving the backup, the Snow Leopard way

Trying to troubleshoot the above, I found the easy answer on an Apple Support forum thread: Just manually copy the old Time Machine Backups.backupdb folder from the old drive to the new.

That's it? That's it. It won't work in Leopard; the Finder's copying process won't replicate some of the under-the-hood file structure details needed for Leopard to recognize the copied folder as a proper Time Machine backup. Snow Leopard, on the other hand, faithfully takes care of those details. The user doesn't need to know the technical details;it just works. 

So that's what I did. No hassles at all. In a nutshell:

1. Connect both disks to your Mac running Snow Leopard.
2. Switch off Time Machine in System Preferences. This is important; you don't want Time Machine kicking in during the copying process!
3. In the finder, drag the Backups.backupdb folder from the old drive to the new to initiate a copy.
4. When copying is complete, open the Time Machine panel in System Preferences and click "Select Disk...".
5. Choose the new Time Machine disk and click "Use for Backup".
6. Turn on Time Machine. Test it out: Activate Time Machine and search for some old files. Confirm that it's making new hourly backups. 

If all seems well, you're done. I now have my old Time Machine backup living (and growing) on a spacious 1 TB disk, with the old 500 GB disk taking on other storage duties.

The above is an example of the many "invisible" improvements made in Mac OS X 10.6 that most users won't even notice, but make things easier all the same. Thanks, Snow Leopard!