Early hard disk drives used a method of encoding called FM (frequency modulation). FM technology is based on timing. To differentiate a 1 from a 0, it measures the time the drive head spends in a magnetized state. For FM to work, it requires every 1 or 0 to be preceded by a timing bit. The early FM drives worked well, but all the extra bits added to the work and slowed the process of data transfer. In order to improve efficiency and speed of the data transfer, the FM was replaced by an improved version that reduced the number of timing bits required. This new technology was called MFM (modified frequency modulation). MFM uses the preceding data bit to indicate whether the current bit is a 1 or a 0, thus reducing the number of timing bits by more than 50 percent.
Another method used to place data on hard disk drives is run-length limited (RLL) encoding. RLL replaces the timing bits with patterns of 1s and 0s that represent longer patterns of 1s and 0s. Although this looks inefficient, the elimination of the timing bits speeds overall performance.
Unless you're working with hard disk drives manufactured before 1989, it is not necessary to know which type of data encoding is used.