The "Dirty Bit" is set by the operating system when changes are made to
the disk but not yet committed, once the change is committed to the disk
(written to the disk) the dirty bit is removed. If there is a file
system error or disk corruption or if something fails before the changes
are committed to the disk (like a power failure) the dirty bit remains
set and on reboot Windows automatically checks the disk for errors, upon
successful completion of the disk check the dirty bit is removed. If
the dirty bit was set by the operating system it cannot be removed
manually by the user.
Try running a chkdsk on the disk while Windows is up and running. Make
sure that there are no open handles on the disk, make sure that there
are no open files on the disk or that no applications have a handle on
the disk, for example make sure that you are not exploring the volume
with Explorer.exe. At the Command Prompt issue chkdsk /f and let the
command run its course.
John