Magnetic disk = hard disk (mechanical device with moving parts.).
Data is represented on the disk surface by magnetised dots. Binary data is represented through two states: 'magnetised' and 'not-magnetised'.
Read-write process -disk spins. Rotation speed - rate of data access for a magnetic disk. Faster the disk spins, quicker data is retrieved. Standard drives spin - 5,400 rpm.
Fragmentation - insufficient contiguous space to store the file. Each part of file will be in a separate location with a link to the next location; file is read by following the links until end-of-file marker is reached. If a disk is nearly full, it will take longer to find space to write file segments, and longer to access a file that is spread across several locations.
Disk unit comprises 1+ double-sided metal platters with a magnetic coating. Platter number determines storage capacity of the disk. Binary data is stored on platters as polarised states. Represents 1/0. Platters are broken into concentric tracks, sectors. Contiguous group of sectors-cluster/block/allocation unit.
Location of each file is recorded in a file allocation record. Allows file to be retrieved, and free space to be identified for new files. When disk is full, files split across blocks that are non-contiguous, file allocation record keeps track of where file segments are and order in which they should be reassembled. If record is lost/corrupted, file data will be present on the disk. Impossible to access it unless record can be rebuilt.
Read/write data, the device has two mechanical arms. End of the arm is a read-write head with a tiny magnet. To reduce wear through friction, head doesn't touch the platter; there's a thin gap between the head and the disk surface. Platters spin - speed of 7,200 rpm. Magnetic tapes allow serial access.
Types:
Hard disk drive
Floppy disk
Magnetic tape