Not Enough Free Disk Space

• To defragment a hard drive, the defragging tool needs free space from which to swap file data to and from. Microsoft recommends at least 15 percent free drive space when defragmenting, and if there isn't adequate free drive space, defragmenting errors may occur. You can fix this by either deleting unneeded files or transferring data to another drive. The Microsoft TechNet website has available a list of ways to free up drive space.
  
  

Network Location

• Although you can transfer files to and from network locations as if they were local hard drives, Disk Defragmenter will not work on network-mounted volumes. You'll need to run the defragging tool locally on the host machine to which the network location points.
  
  

Active Program

• Having active programs may result in defragging errors, because Disk Defragmenter can't defragment active program files. To avoid this, close all open programs before starting the defragging process.
  
  

Downloading, Saving or Copying Files

• Disk Defragmenter cannot defrag disk sectors that are actively being written to. Downloading, saving or copying files will impede the defragging process, and Disk Defragmenter may need to start over. Avoid writing data to the hard drive while the tool is running -- you can schedule Disk Defragmenter to run at specific times when you're away from the computer.
  
  

Disk Error

• Disk Defragmenter can stop working if there are any disk errors. Bad sectors or hardware issues will cause Disk Defragmenter to throw up an error message and stop the defragging process. Microsoft Support recommends running ScanDisk on your drive if this happens.