Reformat and repartition hard drives before using them with Time Machine

A month or so ago I bought an external hard drive to use as a backup disk with Apple’s Time Machine. Previously I used DVD-R:s for backing up my personal data, but I wanted an automated backup system in place. Since Mac OS X comes with Time Machine built in, I decided to give it a try.

Getting started was easy. I bought an external USB drive, hooked it up, reformatted it as Mac OS Extended (Journaled) instead of the Windows file system the drive shipped with, and started Time Machine.

Everything worked fine for a couple of weeks, but then some strange things started happening. Time Machine reported failed backup attempts and sometimes seemed to have problems even finding the backup disk. This was obviously worrying, so I started looking for what could be causing it.

Beware of the Master Boot Record partition type

What I found was that Time Machine does not work well with the Master Boot Record (MBR) partition map scheme that Windows-formatted disks use. According to the Apple support document Mac OS X 10.5: Time Machine stops backing up to external disk, Time Machine wants disks to use either GUID Partition Table or Apple Partition Map.

Fine, but the problem is that to change the partition map scheme you need to erase the disk. Doing that obviously means losing all your backup history up to that point, which I was not interested in.

Luckily I had bought an identical hard drive to backup another Mac, and was able to use the Disk Utility application included with Mac OS X 10.5 to clone the contents of my Time Machine drive to the other drive after changing its partition map scheme. It took several hours, but all seems to be working well and my backup history is intact.

You may be wondering why reformatting the disk did not also change the partition map scheme. Well, the setting for this is hidden in an Options dialog in Disk Utility’s Partition tab, and I never went there when I reformatted the drive. I just clicked on the Erase tab, selected Mac OS Extended (Journaled) as the volume format, entered a volume name, and clicked Erase. This apparently does not change the disks partition map scheme.

Do this before using a disk with Time Machine

When you buy a hard drive that you will use with Time Machine, the first thing you do after connecting it to your Mac should be this (the instructions are for the current version of Mac OS X, which is 10.5.6):

  1. Open Disk Utility
  2. Select the new hard drive
  3. Go to the Partition tab
  4. Choose the number of volumes (normally one) from the Volume Scheme dropdown
  5. Click on the Options… button
  6. Choose GUID Partition Table or Apple Partition Map depending on whether your Mac is Intel-based or PowerPC-based.
  7. Click Apply.

Now the drive should be fully supported by Time Machine and things should work fine.

Why didn’t Time Machine warn me?

A question I asked myself after going through this trouble with reformatting, repartitioning and cloning hard drives is why Time Machine didn’t tell me that it doesn’t fully support the drive I connected. I hope there is a good answer since most hard drives you buy will use the unsupported MBR, and changing it when you’ve been using the drive for a while and start noticing problems is very frustrating and time consuming.

Anyway, now that you’ve read this you’ll know how to avoid the problems I had :-).

Posted on January 15, 2009 in Mac, Apple