A: In Windows 7, BitLocker volume encryption is performed after the installation of the OS.In Windows 8, BitLocker can be applied to a volume before the OS is installed, by using the Windows Pre-installation Environment (WinPE) which can also be leveraged from System Center Configuration Manager 2012 SP1.

When combining the new BitLocker pre-provisioning feature on an empty volume with BitLocker Used Disk Space Only option, BitLocker encryption can be completed in seconds.