I would think that your error message actually referred to running CHKDSK which has an /F parameter to repair the disk. However, you can run the repair from within Windows. Double-click 'My Computer' and right-click on your external drive then choose 'Properties'. Click the 'Tools' tab and then click the 'Check Now' button in the error checking section. In the next dialogue box put a tick in both of the check boxes then click 'Start'. Windows will now scan the drive and repair any bad sectors. Depending on how large your drive is, this can take quite a long time and goes trough 5 phases but when it has finished you should be able to convert your drive to NTFS.