One of the requirements for setting up BitLocker on a computer is that the hard drive must have at least two partitions.  Most Windows 7 and Windows 8 installations will have these two partitions by default, one being the System partition and the other being the Operating System partition.  However, on rare occasions depending on how the Operating System was installed, you may encounter a computer that only has one partition.  To check this, on Windows 7 systems, click the Start button, right-click on Computer, and select Manage.  On Windows 8.1 systems, press the Windows key on the keyboard and the X key together and select Computer Management.  Then in the left hand pane, under Storage, click on Disk Management.

Drive with a single partition.

Drive with two partitions.

If your hard drive only has one partition you can create the extra partition required for BitLocker using the BitLocker Drive Preparation Tool.  This is a command line utility built into Windows.  Open an administrative command prompt (right-click and choose Run as administrator) and type: 

Bdehdcfg.exe –target default

Then press enter.  After the command completes you should see two partitions in Disk Management.

Then restart the machine. 

That’s it!  Your hard drive is now prepared for BitLocker drive encryption.