can't defrag

I have two 80 Gb hard drives in my Mac G4. I use Drive Genius in addition to Disk Utilities to repair/rebuild & defrag from time to time. I defragged one drive no problem, but with the other I was unable to "due to a serious problems found on disk". According to Drive Genius I have to rebuild before continuing. Well I've done that and it still won't defrag. When I verify the drive I get the following message "Error : Volume bitmap shows block 543538 to be free but the catalog shows it to be used. (1106909,543538,6)
Error : Free block count does not agree with the catalog (6853950/6853916)".
I have no idea how to fix this. Any suggestions??

Thanks,
Andrew