BitLocker is a new security feature first introduced with Windows Vista and now available for Windows Server 2008. BitLocker is best known for its ability to encrypt the disk, but it also provides the ability to lock the normal startup process until you supply a personal identification number (PIN) or a removable USB Flash drive that contains the startup key.
Using BitLocker and especially using a PIN or a USB Flash drive for startup can slow the startup process and can require someone to physically be at the server to start it.
Always maintain security for the USB Flash drive that you use for BitLocker. If it gets into the wrong hands, the security of the server is compromised.