How can I fix this?

A small data file such an e-mail message of only a few hundred bytes can take up an entire 32K of disk space. A file that spills just a few bytes over the cluster size will take up another whole cluster. Much disk space can be lost in this way.

You didn't mention which diagnostic test you used, but there are several utilities that can help you with partition organizational procedures. One such utility is Partition Magic.

Look at this link for more information:

http://www.powerquest.com/partitionmagic/

REMOVE SPACES from the pasted URL.

As maxwell edison noted who's diagnostic program were you using?, I would suggest you visit your harddrive manufacturers web site & download their utility to test your drive they all have them. Third party software isn't always very reliable for diagnostics since they have to be so general in nature.

If your running under Window's run a thorough scandisk (make sure fix errors is checked) & defrag then run the diagnostics again.