SCSI drives formatted under different versions of HD SC Setup may show
differing numbers of blocks available, even though the storage capacity
of the drives is theoretically the same. This is due to different methods
used by various versions of HD SC Setup.
HD SC Setup Version 1.1 checks each drive and formats the disk with that
drive's maximum number of blocks. Versions 1.4 and later determine the
approximate size of the drive (20MB, 40MB, or 80MB), then format all
manufacturers' drives with a set number of blocks. On an HD-20SC, the
result is 19,171 blocks. This is the number of blocks available on an Apple
HD-20.
Apple made this change in order to provide an easier method of using tape
backup systems, and to provide a consistent amount of storage on a hard
disk without regard to the computer or drive's manufacturer. Since all
20MB drives formatted with version 1.4 and later will contain the same
number of blocks, it is possible to restore a backup from one drive onto
another type of drive without encountering problems due to insufficient
storage. This will avoid problems that could develop when users compare two
"identical" units and find that they have different amounts of disk storage
available.