Comparing magnetic drives, CD drives, and flash drives.
Magnetic hard drives store information by using an electric field to orient the magnetic field of small spaces to produce binary information. A lot of moving parts and susceptible to damage from dropping and magnets. Common in large, stationary computers. $0.08 per GB
CD ROM are drives that store information by reflecting or absorbing light from a laser. It still produces information in binary bits. Discs can be damaged, and the laser needs to be kept clean. Still some moving parts, but not directly in contact with stored information. A bad CD drive does not damage the data. Common for sharing programs and massive storage (pictures, video). Rewriting a portion only can be difficult. $0.04 - $0.25 per GB.
Flash drive stores information using modified transistors that hold a small charge between the two gates. Memory is stored as a bit by conducting a charge into the oxidized layer or draining it out. Allows for a very small area that can be read in multiple directions - not limited to one direction on a spinning disc. No moving parts so risk of damage is low. Cannot be erased with a magnet. Can be damaged by high levels of humidity (corrosion) and do wear out over time from frequent use. Electrical shock can also corrupt data. Examples include thumb drives and MacBook Air drives. $0.60 per GB.