e/Index mark

Information
has gloss	eng: In computing, an index mark or index track is a physical impression made on a hard disk drive. Its purpose is to indicate the starting point for each track on the hard disk drive. Usually, an index mark takes the form of a hole, gap, or magnetic strip. It also allows the hard disk drive head to quickly move to various spots on the drive.
lexicalization	eng: index mark
instance of	c/Computer storage devices