Unmovable files are files with a physical location on a hard drive that is fixed in the hardware and cannot be moved by normal defragmentation. These are usually system files like the paging file, the NTFS log file or NTFS Change Journal. The Master Boot Record, or MBR, also cannot be moved.

Defragmentation of a hard drive involves reorganizing data into continuous blocks on the hard drive to the extent possible, similar to rearranging loose pages of a book into their proper order to make it easier to efficiently read. Computers usually run more quickly after disk defragmentation. Although most data can be defragmented, the MBR is not movable. It resides on block 0 of the drive and contains information about the location of operating system files.

The paging file, meanwhile, is a file that Windows uses to supplement its physical RAM and increase the computer's performance. It cannot be moved because the computer needs to know where to look for data storage in conjunction with RAM. The file location at the operating system level can be changed, but not the physical location on the drive.

The only way to affect unmovable portions of the hard drive is to reformat them and create new partitions.