Q: What is the Windows 8 BitLocker option called Used Disk Space Only?

A: Typically, when BitLocker is applied to a volume, all space on the volume is encrypted even if that space is empty (which is why a very large temporary file is created to fill up empty space during encryption).

The logic is, even though a file is deleted, it's content is actually still on disk, and so for maximum security, even unused areas of disk should be encrypted to ensure full security.

If you have a brand new hard disk, then there are no concerns about old data and, thus, the encryption of empty space isn't required, so Windows 8 gives the option when encrypting a volume with BitLocker (see screen shot below) to choose how much of your drive to encrypt. For many computers the bulk of space is empty, so using this new option will speed up BitLocker encryption times.

Win8BitLockerSelectsml

From the command line, use the Enable-BitLocker cmdlet with the -UsedSpaceOnly switch.

The same switch can be used with Manage-bde.exe as well (with Manage-bde a shortened version, -used can also be specified instead of the full -userspaceonly).