I can only speak for "other" unix systems. There is a filesystem flag that indicates if a disk is mounted or not. On system boot this flag is looked at for each filesystem that is automatically mounted. If the flag indicates the filesystem was unmounted properly, fsck is not run. If the flag indicates the filesystem thinks it is mounted, then fsck is run. So the answer to your question is: fsck is run at boot only if the system crashed. Again, I am speaking for the other flavors of unix that I have worked with. I am NOT an OSX expert. Brian