by , 7:30 AM EDT, August 16th, 2006

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. Apple's 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.

Here's how to format a drive with GPT:

• Launch Disk Utility. It's in Applications/Utilities/Disk Utility.
• Choose the drive you want to format from the drive list on the left. Be sure to select the drive's description instead of its name.

Be sure to select your drive's description before clicking Options.

• Click the Partition tab. If you don't see the Partition tab, you selected the drive's name. Click on the description just above the drive name to reveal the Partition tab.
• Click the Options button.
• Select GUID Partition Table.

Choose GUID to make your drive bootable.

• Click OK.
• Click Partition.

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 don't need to worry about this with internal drives, either. GUID is the default selection when you format drives installed inside your Mac.

if you have ideas for Mac related tips that you think other TMO readers might find helpful.