You've got to secure your data. With OS X 10.7 Lion, Mac users now can encrypt external USB hard drives, Flash drives and SD cards.
The Encryption provided byt OS X Lion is called FileVault2. It uses full disk, XTS - AES 128 encryption to keep your data secure.
It'll format and encrypt any media the Disk Utility supports. Here's how to encrypt your external media via OS X 10. 7 Lion.
Inside the Application folder on your Mac, double-click on Utilities.
Locate and double-click Disk Utility to launch it.
In this example we are encrypting a USB thumb drive so we click on it inside of Disk Utility.
Next, click on the Erase tab.
Choose the format you want to use for encryption. Here there are two. I recommend selecting Mac OS Extended (Journaled, Encrypted).
Now click Erase to start the formatting process.
Warning: This erases all contents on the target drive. Make sure you back up all data you need before you proceed.
Enter a password. The more complicated the password, the better the protection. Use a mixture of upper and lowercase letters, numbers, and symbols.
Hit Erase once more. This starts the formatting process. Click Cancel to stop at any point.
Once the process is complete you’ll be able to use this or any other encrypted external drive just like before except for one thing. If you eject the drive and later re-attach it to your Mac you’ll be prompted to enter the password you selected when you formatted it.
Click Unlock and your drive is immediately mounted and available to you and your Mac. The contents are accessible until you eject the drive.