Removes the bad spots from the volume bitmap of available free storage. The Finder does physical copies only on 1594 block disks as an optimized method of disk copying. In practice, this means that the first 50 sectors and the second to the last sector of a 1440K FDHD disk must be good. Even if there is an error somewhere in the alternate MDB's track, the algorithm does not reject the disk if the sector it is on is useable. Most of these were successfully spared using the new Disk Initialization Package.
Interior errors seem to be rare.
One explanation for the high proportion of errors on side one tracks 75-79 might be that Apple now parks the heads over that area before ejecting a disk, so when a disk is inserted the heads come down there, possibly touching and scraping the media. Side one is probably more vulnerable both because the diameter of a given track is smaller on that side and because the head faces up and therefore easily collects dust. In particular, this includes disk utilities such as Disk First Aid, that repair and reorganize HFS volumes. Note that these same applications also need to be revised to correctly handle System Software 7.0 aliases. Apple has a version of Disk First Aid that supports both sparing and aliases, and it ships as part of System Software 7.0.