Here's a quick and easy way to encrypt external disks in OS X Mountain Lion.
1. Right click on the external disk.
2. Click on the "Encrypt DISK NAME" option in the menu.
3. Choose your password, and your password hint.
4. Click the Encrypt Disk button.
Make sure that you do not lose your password, or you will lose access to your data on the encrypted disk. Encryption can be a powerful tool to protect your OS X Mountain Lion data.
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