Most hard drives are in the megabyte range.
Also see Gigabyte. MFM uses a fixed length encoding scheme. Flux reversals on the disk always will be evenly spaced in time so that the beginning of one bit can be separated from another. This type of scheme allows even single bit errors to be detected easily and corrected by the controller electronics. MFM is a modified version of FM which does away with the need for timing signals allowing it to pack twice as much data on the same drive.