Types of External Memory

• Magnetic Disk

—   RAID

—   Removable

• Optical

—   CD-ROM

—   CD-Recordable (CD-R)

—   CD-R/W

—   DVD

• Magnetic Tape

Magnetic Disk

• Disk substrate coated with magnetizable material (iron oxide…rust)


• Substrate used to be aluminium


• Now glass

—   Improved surface uniformity

–     Increases reliability

—   Reduction in surface defects

–     Reduced read/write errors

—   Lower flight heights (See later)

—   Better stiffness

—   Better shock/damage resistance

Read and Write Mechanisms

• Recording and retrieval via conductive coil called a head


• May be single read/write head or separate ones


• During read/write, head is stationary, platter rotates


• Write

—   Current through coil produces magnetic field

—   Pulses sent to head

—   Magnetic pattern recorded on surface below

• Read (traditional)

—   Magnetic field moving relative to coil produces current

—   Coil is the same for read and write

• Read (contemporary)

—   Separate read head, close to write head

—   Partially shielded magneto resistive (MR) sensor

—   Electrical resistance depends on direction of magnetic field

—   High frequency operation

—   Higher storage density and speed