EXTERNAL MEMORY Magnetic DiskA disk is a circular platter constructed of nonmagnetic material, called the substrate,coated with a magnetizable material.-substrate: It can be aluminum or aluminum alloy and glass material.!ene"its o" glass substrate: Improvement in the uniformity of the magnetic film surface to increase disk reliability A significant reduction in overall surface defects to help reduce read-write errors Ability to support lower fly heights (described subseuently! "etter stiffness to reduce disk dynamics #reater ability to withstand shock and damageMagnetic Read and #rite Mec$anisms:$ata are recorded on and later retrieved from the disk via a conducting coil named the$ead%%ostsystems has two heads namely& &rite $ead ' read $ead%(he read head consists of a partially shielded magneto resisti'e (MR) sensor% (he %) material has an electrical resistance that depends on the direction of the magnetization of the medium moving underit. "ypassingacurrent throughthe%)sensor,resistancechangesandaredetectedas 'oltagesignals. (he%)designallows$ig$er-"re*uencyo+eration, whicheuatestogreater storagedensities and operating speeds.Data Organi,ation and -ormatting:The head is a relatively small device capable of reading from or writing to a portion of theplatterrotating beneath it. This gives rise to the organization of data on the platter in aconcentric set of rings,calledtracks. Each track isthe same width as thehead.There are thousands of tracks per surface.Ad*acent tracks are separated by ga+s.(his prevents, or at least minimizes, errors due to misalignmentof thehead or simply interference of magnetic fields. (here are typically $undreds o" sectors +ertrack,and these may beofeither"i.edor'ariable lengt$. In most contemporary systems,"i.ed-lengt$ sectors are used/ &it$ 012 bytesbeing the nearly universal sector size. (o avoid imposing unreasonableprecision reuirements on the system,ad3acent sectors are se+arated by intratrack (intersector) ga+s%The information that is scanned at the same rate by rotating the disk at a fxed speed, iscalledconstant angular 'elocity (+A,!.-ad'antage o" 4A5: (he advantage of using +A, is that individual blocks of data can be directly addressed bytrack and sector.-disad'antage o" 4A5: (he disadvantage of +A, is that the amount of data that can be stored on the long outer tracks is the only same as what can be stored on the short inner tracks.