eeprom and flash memory media have individually erasable segments, each of which can be put through a limited number of erase cycles before becoming unreliable. this is usually around 3,000/5,000 cycles but many flash devices have one block with a specially extended life of 100,000+ cycles that can be used by the flash memory controller to track wear and movement of data across segments. erasable optical media such as cd-rw and dvd-rw are rated at up to 1,000 cycles (100,000 cycles for dvd-ram media).