Non-Data Overhead I
Whether using a block or a sector organization, some space on the disk is taken up by non-data overhead. i.e., information stored on the disk during pre-formatting.
On sector-addressable disks, pre-formatting involves storing, at the beginning of each sector, sector address, track address and condition (usable or defective) + gaps and synchronization marks between fields of info to help the read/write mechanism distinguish between them.
On Block-Organized disks, subblock + interblock gaps have to be provided with every block. The relative amount of non-data space necessary for a block scheme is higher than for a sector-scheme.