Which storage system consists of magnetic material and may require an external power source to run?

A) CF card

B) UDO

C) SD card

D) HDD

The CF card is known as compact flash memory card and can be directly connected in portable devices. UDO is known as Ultra Density Optical and it is not a backward compatible. SD card is known as Secure Digital card and also used for portable devices. While HDD is known as the Hard Disk Drive and it is used to store large sizes of files. HDD needs an external power source to run.

The answer is letter D. HDD.