Have you ever wondered why your disk has no free space left even though you should have at least 300 GB left? Do you have Bitlocker enabled? If you answer yes, this might just be the reason for this.
As you might know, when you delete a file it actually doesn’t get physically removed from your hard drive. This is one of the reasons why it’s possible to restore a file even though it has been removed from the file system. Since free space might be crowded with old deleted files and the like, Bitlocker will also encrypt this part of your hard drive for security reasons. When encrypting, Bitlocker will create a large file that will be ~6 GB smaller than what is left on your drive. This file is filled with “noise” ¬†of ASCII code. The reason for leaving 6 GB of free space this is that you probably will want to use your drive while encrypting the rest of your files.

You can read more about this on the TechNet blog: