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Wikipedia said:
Instead of the hard disk which is used in the iPod classic, the nano uses flash memory. This means there are no moving parts, making the iPod nano resistant to skipping due to sudden movement.
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All iPods and iPhones aside from the iPod Classic use flash memory, which is a form of EEPROM. The classic still uses a Hard Disk. Solid State Drives are still wayyy too expensive, and use SRAM or DRAM, which is different than Flash Memory. If you want an idea of the cost of an SSD, look at the MacBook Air with this option versus the Hard Disk based version.
 
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