After lots of usage, electromigration might cause damage to the
gate oxide irreversibly. The gate oxide might have a fixed charge,
or there might be induced interface charges... Something like
that...
In addition, the manufacturers of the flash memory modules
usually guarantees the assemblers a maximum of 10000 read write
cycles. The assemblers will assemble the modules on the PCB and
test it out.
10000 might seem a lot, but if you transfer movies continuously,
you will use them out very fast.
"..............a flash device that is rated at 100,000 write
cycles doesn't mean it wears out after as many uses. It means it
can write 100,000 times "to every single (memory) cell within the device........"
http://pspupdates.qj.net/Samsung-it-s-virtually-impossible-to-wear-out-a-flash-chip/pg/49/aid/114440