RAID : Redundant Array of Independent Disks. Hard disks (HD) usually have multiple disks (2 or 3) called platters. HD platters are made of glass or glass-ceramic. Platters are coated with magnetic substances. A thin film of iron oxide is located over the platter with the help of electroplating mechanism called sputtering. The thin film media is virtually uncrashable and looks like a mirror. HD runs at 7200 rpm, 20 times faster than floppy disks.Speeds can be 10,000 or 15,000 rpm. Data transfer in Seagate Cheetah 73 LP HD (15,000 rpm) is 51.15 Mb/s. 900 plus sectors per track. Sector capacity of 512 bytes. Track density in HD is 5000 per inch. Seagate HD is the leader in the number of hard disk sales.

Sony introduced the first 3.5" floppy disk in 1983. The 1.44 Mb (1440 kb) floppy disk has 80 cylinders and two heads. Each track has 18 sectors. Floppy controllers use IRQ 6 (Interrupt Request). Floppy drives use 4 pin female connector for power (Pin 1 : Yellow-12 V, Pin 2 : Black-ground, Pin 3 : Black-ground, Pin 4 : Red-5 V) and a 34 pin data cable. Floppy disks use FAT system. Metal detector can damage floppy disks.