Q: On my BitLocker encrypted D: drive the Windows Explorer properties tell me there is hardly any free space left, even though it should at least still have 200 Gb of space. What’s causing this?
A: This is caused by normal BitLocker behavior and means that BitLocker is still in the process of encrypting your drive. When the encryption of the volume is paused or completed, the amount of available free space on your drive will revert to its normal size.
When you delete a file from the Windows file system it doesn’t actually get physically removed from your drive. This will allow you to restore the file even though it has been removed from the file system. Because of this free space might be full of traces of valuable old deleted files and Bitlocker will also encrypt this part of your hard drive - just to make sure, for security reasons.
To be more precise: when encrypting a volume, Bitlocker will create a large “wipe” file that is about 6 GB smaller than what is left on your drive and that will span close to all free space on your drive. BitLocker will fill this file with “noise” of ASCII code and will use it to speed up and simplify the encryption process (deciding on what and what not to encrypt on a fragmented file system is a complex task). When the encryption of the volume is paused or completed, BitLocker will delete the wipe file and the amount of available free space will revert to normal. The reason for leaving 6 GB of free space this is to allow you to use your drive while BitLocker is encrypting the rest of your files.
You can find more technical nuts and bolts on this phenomenon in the following TechNet blog: http://blogs.technet.com/b/bitlocker/archive/2006/07/08/unallocated.aspx