The Disk Defragmenter tool is a built-in Windows OS utility that defragments drives, by running a low priority task in the background without affecting the performance of the compute. You can also change the behavior of the Disk Defragmenter tool, and select the partition that you want to defragment.

To open it, type defrag in Windows 10 taskbar search and select Defragment and optimize Drives from the search results. Now when it opens, you will see that, apart from your regular drives, it even displays a hidden partition volume, that is not mounted, with the following name:

\\?\Volume{Number String}

Says KB952910 – This behavior is by design:

After installing Windows, when you display the list of disks for defragmentation from in the Disk Defragmenter tool, hidden partitions that are not mounted are also displayed.

Can you defragment hidden, not mounted partitions?

If you wish to defragment it, you can do so. There is no problem when disk defragmentation is performed for volumes named “\\?\Volume{Number String}, on a computer that is running Windows 10, Windows 8.1, Windows 8, Windows 7 or Windows Vista.

Some points to know about the Disk Defragmenter Tool:

• Disk Defragmenter does not defragment files in the Recycle Bin.
• Disk Defragmenter will also not defragment files which are in use.
• Disk Defragmenter does not defragment the following files: Bootsect DOS, Safeboot fs, Safeboot csv, Safeboot rsv, Hiberfil sys, Memory dmp and the Windows Page file. However using the -b parameter, will optimize the boot files.