The dcheck utility checks for bad blocks on a disk by attempting to read every block on the drive.

When you specify the -m option, dcheck removes any bad blocks from the disk allocation bitmap (/.bitmap).

If it finds the file /.bad_blks, dcheck updates the bitmap and recreates the /.bad_blks file. You can run dcheck a few times to increase your chances of recognizing bad blocks and adding them to the /.bad_blks file.