Both are non-volatile and reliable forms of data storage and most of the portable MP3 devices are either flash based or hard disk based. Traditionally, hard disk drives were the primary way of data storage and were used in most of the early MP3 playing devices. Flash devices are solid memories with no moving parts, relying instead on storing small amounts of electrical charge in semiconductor transistor structures. Hard drives, on the other hand, rely on a mechanical system consisting of a spinning platter that retains information in magnetic storage elements.

Both flash drives and hard drives have experienced phenomenal density growth, with hard drives doubling their storage capacity every two years while flash devices doubling their density every year. It’s speculated that flash based memory devices will eventually replace hard disk memory deices. However, in the longer term it is not clear which technology will take over, but in the short term hard drives and flash will compete within certain market segments.

Apple iPod Nano is a flash based MP3 player while Apple uses a hard disk based memory in iPod video players. The biggest limitation of flash-based MP3 players is that they yet do not have a great deal of storage capacity. For larger storage needs, hard disk-based memory is used. These can have storage capacities in the tens of thousands of songs, but sacrifice the ultra compactness of flash-based players.