Strange Ghost problem

Actually the problem seems to have gone away, but I am reporting for any comment viewers wish to make.
I was trying to use Ghost 9.0 to make a Backup of my C:\ drive. Ghost reported that the data on C:\ was about 38 GB in size, whereas it actually was less than half that size.
I rebooted and all was OK. Ghost now saw the data on C:\ as its correct size.
Because of the initial report of 38GB, the destination drive was judged to be too small and compression was recommended.