Partition Schemes


A partition scheme is the format that is used to describe how the disk is divided up. The partition scheme will say, as a minimum, where each partition lies on the disk, its size and the type. They differ in how exactly this data is stored on the disk.

With the schemes below, the scheme used will affect whether a particular partition is bootable, and/or its compatibility with Windows.

There are three different partition schemes that Mac OS X supports:

Boot Camp and Windows Compatibility

Intel Macs support dual booting using Boot Camp. This works by having both the GUID partition scheme and the Master Boot Record scheme on your disk at the same time. When repartitioning disks like this, care must be taken to ensure both schemes are kept consistent. iPartition will keep both schemes correctly updated; other software might not be aware.

If you want partitions to be visible in Windows, you need to make sure the “Visible in Windows” partition property in the Inspector is checked. iPartition will only allow up to three partitions to be visible in Windows. When creating new partitions iPartition will not make them visible by default.

Converting between schemes

iPartition has the capability to convert between schemes. Choose the Change Partition Scheme menu option or change it via the drop-down box on the “Disk” tab of the Inspector. If you see an error then it might be because there is not enough space to perform the conversion (due to the fact that some schemes take up slightly more space than others). If this is the case, you may need to shrink a partition to make room (if you shrink it by too much, you can grow it afterwards and iPartition will only change the size of the partition once).

If you change between schemes, you may lose some system partitions which mostly exist to make things bootable. OS X doesn’t need any system partitions for booting (except on very old Macs) so this will not normally be an issue. If you boot OS 9 from the disk, then you should not change the scheme.

Installing OS X on disks with GUID partition schemes

The Intel installer for Mac OS X will not allow you to install onto partitions that do not have 128 MB of free space immediately following a partition. However, once installed, Mac OS X will work happily without this reserved space (but we don’t recommend that you delete it). By default iPartition will reserve this amount of space after every partition it creates but you can choose not to by unchecking the appropriate flag in the Inspector. The Intel Mac OS X installer also requires there to be an EFI System Partition on the disk that is exactly 200 MB in size. If you’re creating a GUID partition map from scratch, you might find it easier to create it using Disk Utility as this will create the EFI System Partition for you. You can manipulate the partition map in iPartition later.

Disks that are bootable on both PowerPC and Intel Macs

It is possible to have a disk that is bootable on both PowerPC and Intel Macs. This section discusses how to make a disk that has two partitions, one that will boot PowerPC machines and one that will boot Intel machines. It is actually possible to have a single partition that will boot both, but the procedure to do this is more complicated (check the Internet if this is what you wish to do). The Leopard version of OS X will boot both Intel and PowerPC machines without any problems and so this section is not relevant if you have Leopard.

You will need a disk that is formatted with an Apple Partition Map, but the Intel Mac OS X installer will not allow you to install OS X on an Apple Partition Map formatted disk. So, first create a disk with the GUID partition scheme and then you can install the the Intel version of Mac OS X onto the disk. Then, using iPartition, convert the scheme to an Apple partition scheme. Now you can install the PowerPC version of Mac OS X and you should be able to boot on both Intel and PowerPC machines.