The System Restore process introduced in Windows XP regularly creates backups of your data using a feature called Volume Shadow Service (VSS). This feature does a lot of work in the background to make sure that you never lose any data. Unfortunately, during defragmentation, it can try a little too hard and create extra backups that you just don't need.
You can find out more about this service here: https://docs.microsoft.com/en-us/windows/win32/vss/volume-shadow-copy-service-overview
So, if you find that you have lost free space after defragging (and this goes for all defrag tools, not just Defraggler), VSS is probably the cause.
However, when they are made by mistake, such as during defragmentation, these copies can be safely deleted to regain free space.
To delete volume shadow copies manually and safely:
- Start the Disk Cleanup program (normally located in the Accessories folder, or you can search for it from the Start menu).
- Select the drive where the shadow volume copies are located (typically, the drive where Windows is installed), and click 'OK'.
- On the 'More options' tab, in the System Restore and Shadow Copies section, click 'Clean Up'.
- Disk Cleanup prompts you to confirm the action. Click 'Delete'.
To disable the Volume Snapshot Service:
Defraggler allows you to disable VSS during the defrag process, automatically reenabling it afterwards.
To do so, open Defraggler and click 'Settings' > 'Options' > 'Advanced' > check 'Stop VSS when defragmenting NTFS volume', as illustrated in the image below.