The free space on hard drive usually contains the contents of the files that were previously deleted using standard Windows system commands. When you delete files manually in Windows Explorer, Windows just mark them as deleted in File Allocation Table, even you can not see them, the deleted files are still on your hard drive, there are many tools available which can let others easily recover your sensitive files.
MFT Free Space
Most users use NTFS file system rather than FAT32 file system, since it supports large drives, offers better performance and encryption of files & folders. Master File Table (MFT), which contains an index of files on NTFS drives. Due to Windows deletes and overwrites files & folders, the MFT could accumulate free space which may contain previous remnants of index pointers to deleted files & folders.
Drive Wiper permanently erases all the data on the free space and ensures that deleted files will not be recovered by any third party software. Note: It is strongly recommended that you close all Windows Explorers, browsers and applications before running Drive Wiper.
1. The larger the free drive space you have, the longer the wipe cycle will take. Wiping free space will significantly increase the amount of time required for cleaning process, we recommend that you perform this operation only when your computer is idle.
2. Due to Secure Wipe will overwrite all the free space on your drive, Windows may warn that drive is full. Please ignore this warning, Secure Wipe will free up the drive space after wiping process is complete.
3. For Windows XP and previous version, this operation may cause the System Restore feature to discard all of your existing Restore Points and create a new Restore Point.