Encryption and Your MacBook
The Security & Privacy pane of your MacBook's System Preferences is particularly important to laptop users because it allows you to encrypt your hard drive. Encryption prevents just about anyone from accessing (or even identifying) any of the files you’ve stored on your system. The robust encryption provided by Lion will certainly stymie just about anyone but the NSA and FBI.
In System Preferences, click Security & Privacy, click the FileVault tab, and then click Turn On FileVault. If your system has multiple user accounts, each user must enter her password to be able to access your encrypted drive (that’s because FileVault uses your login account password as your primary encryption password.)
Copy down the recovery key that Lion displays; then click Continue, and Lion takes care of automatically encrypting and decrypting files as necessary.
To take full advantage of an encrypted hard drive, you need the proper login mode. Think about this possible security backdoor: From the Users & Groups pane, you’ve set your laptop to automatically log you in every time you boot your MacBook.
This is the very definition of Not Secure because your login account password automatically decrypts the data! Therefore, make sure that you actually have to log in to access your account.