Prepare Your Hard Drive - Take Control of Backing Up Your Mac - page 67

scheme affects the entire drive, regardless of how many partitions it
has or how those partitions are formatted.
The majority of hard drives are configured at the factory to use the
MBR scheme, because that's the norm on Windows. In most cases
that's fine—if you plug such a drive into your Mac, it will most likely
work as a backup drive without any intervention. However, it's worth
noting that Time Machine can't use volumes larger than 512 GB on
an MBR-partitioned drive. (That's because Time Machine requires
the Mac OS Extended format, also known as HFS+, and HFS+ volumes
can't be larger than 512 GB on an MBR-partitioned drive.)
You normally need not worry about this; if you select a disk to use
as a Time Machine destination and it's partitioned using the MBR
scheme, Mac OS X will offer to repartition it for you as a GPT disk
automatically. But, if you don't want to use the entire disk for Time
Machine backups—for example, if, as I suggest, you want to divide the
disk into a partition for Time Machine (or other versioned backups)
and a partition to hold a bootable duplicate—then you should manually
repartition the disk before handing it over to Time Machine. As you
do, you should check the partition map scheme, because changing
it requires erasing all the data on the disk; that's obviously something
best done before you've copied any of your personal files onto it.
Although there are a couple of ways to check your drive's partition map
scheme, I recommend using Disk Utility—and then just leaving it open,
because you'll be using it to format your drives in just a moment (see
Configure Your Drive ).
Follow these steps:
1. Open Disk Utility (in /Applications/Utilities ).
2. In the list on the left, select your external drive. (The drive may
have one or more additional icons, representing individual volumes,
indented underneath it; select the topmost icon for that device,
which represents the drive as a whole.)