This post talks about how you can securely wipe the unused space on Windows.
Why you might want to clear unused space?
If you have recently deleted files you want to keep safe from recovery you will want to securely erase it.
It is important to erase them securely as deleting files only marks them as removed. They may still exist on the hard drive and be accessible to recovery tools.
Sanitising your free space
Windows includes a useful tool which works by overwriting your free space. First all the space is written with 0x00, then with 0xFF and then finally with a random character.
Before running this tool it is recommended to close as many programs as you can, so temporary files can be overwritten.
To clear all the empty space on the C drive, you can run the following command.
The final character is the drive letter that you want to clear, so this can be run across any drive.
Please remember that this only operates on the free space on your drive. This means that if you are using it at the time any non-free space will not be overwritten. Many programs will temporarily store files on the hard drive which may contain sensitive information. This is why it is recommended to close all programs not in use.