Nearly all 1/2" tape formats today are a single hub design, whereby the tape is wound onto a single hub, entirely within the external shell and presents one end for the tape drive to engage and thread into the tape drive for reading or writing. The mechanical design of this loading mechanism varies between different types of tape media and is a common source of failures. Usually the tape cartridge incorporates a switch that can be set to permit or forbid writing of data to the tape.