Windows 7 Bitlocker-to-Go allows you to encrypt information on an entire volume. Even if the volume is a removable drive or USB memory key. To support the use of your portable devices, XP SP3 and Vista SP1 can open the encrypted key.
When you allow the key to autoplay, an application on the unencrypted portion of the drive brings up the BitLocker –to-Go reader and asks for the password. If you explore the device without using the reader, you will only see the unencrypted portion of the drive containing the reader software. Also, if you look at the properties of the drive, it is completely filled. The encrypted side will reserve all the space on the volume, minus the small portion for the reader.