Windows Vista was the first to introduce BitLocker, full-disk encryption. With server 2008 and Vista Service pack1, we were given the to encrypt other non-boot fixed drives in addition to the boot partition. In Windows 7 (both the client and Server versions), you now the ability to protect removable storage devices, such as USB-based hard drives, flash devices, and other Microsoft is call this feature "BitLocker To Go".
BitLocker to GO is only available in the Enterprise and Ultimate versions of Windows 7, to create the protected storage. Once this is added to a storage device, that device can be used normally with any version of Windows XP , Vista, and Windows 7. So now you can protect external hard drive and flash drives.