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When choosing and configuring an external hard drive for backing up your Mac, it’s best to start with a new, clean, external hard drive. You don’t want to use an older hard drive that may fail, and if you use a new drive you can get one that closely matches the size of the disk images you’ll be creating. It’s a waste of money to purchase and devote a 200 GB drive to back up, at most, a 50 or 100 GB disk image (if that's all you plan to use it for).
Once you’ve selected a drive you may need or want to partition and format it. Partitioning will let you delegate an exact portion of the drive just for the disk images you create, and each time you create a new image, you can replace the existing one. You can create additional partitions for other data such as music, video, documents, and pictures. You can even create a separate partition for restore points. Although you don’t have to perform this step, it’s ultimately best to take the time to do it.
To prepare an external drive for backing up your Mac:
1. Connect the external drive to your Mac using its FireWire or USB connection.
2. Go through any data on the external drive and delete anything you no longer need.
3. Copy what’s left to your Mac’s desktop. Because the next step is to format the external drive, you’ll want to make sure you have backed up your back up.
4. Open Finder, Applications, Utilities, and Disk Utility.
5. Select the external drive in the left pane (not a partition of that drive).
6. Click the Partition tab.
7. To create two partitions, click Split. Click Split again to create three.
8. Select any partition to resize it and name it, using the choice boxes provided.
9. To create each partition, select it and click Partition. Click Partition again to verify. Note that clicking to verify will partition and format the drive.
10. Select each partition. You will also see the partitions on your Desktop and in Finder.
11. The drive is now ready for backup.