Formatting an external drive to use as a boot volume for Intel-based Macs is easy to do, but if you miss one crucial step, your efforts will be wasted. Appleis new Macs require the GUID Partition Table (GPT) scheme for bootable disks instead of the older Apple Partition Map (APM). The problem is that Disk Utility formats external drives with APM by default.
Hereis how to format a drive with GPT:
- Launch Disk Utility. Itis in Applications/Utilities/Disk Utility.
- Choose the drive you want to format from the drive list on the left. Be sure to select the driveis description instead of its name.
- Click the Partition tab. If you donit see the Partition tab, you selected the driveis name. Click on the description just above the drive name to reveal the Partition tab.
- Click the Options button.
- Select GUID Partition Table.
- Click OK.
- Click Partition.
 Be sure to select your driveis description before clicking Options. |
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 Choose GUID to make your drive bootable. |
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This is necessary only if you plan on using an external drive as a startup disk, and it works with FireWire and USB 2.0 drives. You donit need to worry about this with internal drives, either. GUID is the default selection when you format drives installed inside your Mac.
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