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GPT vs. APM and Using Bootable External Drives

Another (in some ways larger) difference is that Intel Macs use a different partitioning scheme to create bootable hard drives than earlier Macs. The partition scheme lays out the format in which the physical sectors of a disk are ordered into logical blocks. These logical blocks then serve as the basis for creating individual partitions on the disk.

Do not confuse the partition scheme with partition formatting. Formatting a partition makes it available to the operating system to store data and determines the way logical blocks of data are created and used to store data within an existing partition. Intel Macs support all the same formats as their Power PC counterparts: HFS+ (Mac OS Extended), HFS+ with Journaling, UFS, and so on.

Intel Macs rely upon a new firmware technology called Extensible Firmware Interface (EFI), which requires that a hard drive use a new partition scheme called GPT (GUID Partition Table) instead of Apple’s older APM (Apple Partition Map). The differences between the two relate specifically to how the first blocks of data on the physical disk (which describe the partitions on the disk) are laid out. Because EFI and GPT are designed for use in non-Apple PCs as well as Intel Macs, GPT needs to support a level of backward compatibility for the PC Master Boot Record (MBR).

To confuse matters even more, when you connect and partition an external drive, the Apple Disk Utility uses the APM partition scheme by default. Thankfully, as of Mac OS X 10.4.3, Disk Utility has included an Options button in the Partition pane that enables you to specify a partition scheme including GPT, APM, and MBR (to enable use as a bootable drive for PCs). Using a GPT partition scheme prevents the external drive from being bootable on a Power PC Mac, however (and Macs running a version of Mac OS X prior to 10.4.2 cannot even mount the drive).

This creates a number of issues and becomes even more confusing when you consider that the Intel Mac OS X Install DVD is actually formatted using APM. Although there are ways to create a universal external boot drive, they are somewhat separate from the topic of this series. Also, because these are unsupported approaches, you might be best served in a production environment to create two separate external boot drives, if needed.

If you want to learn more about this issue, additional information is available from Jonathan "Wolf" Rentzsch and at the Apple Blog.

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