Information Technology Reference
If you'll use your backup drive with both Intel-based Macs and
older, PowerPC-based Macs, use APM. But be aware that the
maximum size of an APM volume, regardless of the total capacity
of your hard drive or disk array, is 2 TB, and APM-formatted drives
restrict certain activities, such as updating firmware, when used for
booting Intel-based Macs (see the sidebar Partition Scheme and
Bootability just ahead).
For now, simply keep in mind which scheme you chose—MBR, APM,
or GPT. You'll apply it, if necessary, in a few moments.
Partition Scheme and Bootability
Despite Apple's claims to the contrary, Intel-based Macs can boot
from hard disks formatted using the Apple Partition Map (APM)
scheme, which was the norm on PowerPC-based Macs for years.
The catch is that the Mac OS X installer refuses to recognize such
disks as a valid destination and instead requires you to reformat
the drives with Disk Utility to use the new GUID Partition Table (GPT)
scheme. Luckily, however, you needn't worry about any of this when
making backups. If you use one of the utilities described in this topic
to create a duplicate from your Intel-based Mac onto an external
drive, it will be bootable even if the volume uses APM.
Decide How Many Partitions to Make
Wait, didn't we already decide this? Well, yes. Back in Understand
Joe's Basic Backup Strategy , I described how to partition an external
drive into two volumes—one each for duplicates and versioned
backups. However, in some cases you might want to have just one
partition, or more than two:
Time Capsule: If you're using a Time Capsule to store your
versioned backups, you'll be using the external drive just to hold
your duplicate, so it may need only one partition.
Lots of data: If you have so much data to back up that you can't
fit two adequately sized partitions on the drive, then you'll stick
with one, using separate drives for duplicate and versioned backups.