Apple's FileVault is great if you want or need to secure your entire home folder. Unfortunately, FileVault does not encrypt your data on a second hard-drive or a removable drive. If, like me, you use an external portable firewire hard-drive when traveling, you can you Apple's Disk Image Encryption to securely encrypt the data on your removable media.
To create a new "Disk Image" with Encryption
Open the Disk Utility under \applications\utilities.
Click on New Image
Type in the new image name,
Set the encryption to AES-128 bit
Pick an image size appropriate for your removable hard-drive
The image will show up as a volume on your desktop.
Now drag the new image to your portable hard-drive and you are done. All file placed inside the new volume will be automatically encrypted and secured.
You can move files to or from an encrypted disk image as easily as you can from a non-encrypted disk image.