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Memory cards, like all electronic devices, have an estimated lifespan. This is usually called MTBF (Mean Time between Failures). An example of the MTBF for various SanDisk products are as follows: Secure Digital and miniSecure Digital cards: 1,000,000 hours MultiMedia Card: 1,000,000 hours CompactFlash: 1,000,000 hours
SanDisk also sells "Industrial Grade" versions of these flash cards that have a longer lifespan.