Hans,
I'll point you to the man page for
fsck_hfs . Note under
BUGS: "fsck_hfs is not able to fix some inconsistencies that it detects." fsck_hfs is the program Disk Utility and fsck uses to perform a file consistency check on a HFS+ volume. I believe fsck_hfs is designed to fix only certain problems within the file system. This makes to program safe to use. It will not further damage the file system as other third party utilities can.