Disk Manager reports incorrect disk usage a while after the operating system starts in Windows Server 2008 or in Windows Vista
On a computer that is running Windows Server 2008 or Windows Vista, Disk Manager may report incorrect disk usage a while after the operating system starts. The Disk Free Space value is reported to be larger than the total disk space.
For example, assume that you install a 900 GB (gigabyte) hard disk on a computer that is running Windows Server 2008. In this scenario, Disk Manager reports the correct disk usage after you restart the computer. However, Disk Manager reports an incorrect disk usage after a while. The incorrect disk usage may include a Disk Free Space value that is over 1500 GB.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB981891.