In's 52-week course in basic computing. Click here for a complete index of Computer 101 previous lessons. Unless you do something to it, disk-based memory is permanent, even when the power is off. Different types of disk-storage media perform different tasks. Hard disks, for instance, are designed to be fast, capacious and easily modified.
Floppy disks are slower, cheaper and removable.
CD-ROMS are removable and cheap to duplicate. And there's a whole universe of secondary disk devices that offer trade-offs of speed, price and capacity. What all disk-based computer media have in common, though, is the way they are organized. Unlike records, they are random-access storage devices. Think about it for a minute and you'll see why this makes sense.