Flash memory does wear out eventually.
Flash memory is an adventure in packing as much functionality into as small a space as possible. One bit of storage in static RAM is made of six transistors. One bit of storage in dynamic RAM is made of a capacitor with a mosfet controlling whether charge enters or leaves. One bit of flash memory is made of a single mosfet with two gates, one sitting above the other:
The blue material is P-type silicon, the red material is N-type silicon, the grey is silicon dioxide (glass, an insulator), and the yellow parts are the metal contacts. The S contacts are the sources, the D contacts are the drains, the G contacts are the gates, and the W contact on the flash transistor is the read/write input. That blob of N-type material completely surrounded by glass is the flash mosfet's second gate.
In both cases, putting a positive charge in the gate pulls electrons toward the surface of the P-type material, producing a conductive channel between the N-type blobs at the source and drain.
If you apply a relatively high voltage to the read/write contact of a flash mosfet, electrons will enter or leave the encapsulated chunk of N-type material by means of quantum tunneling. Once you remove the high voltage, that charge remains trapped, but it continues to have an influence on the P-type material. If you trap a positive charge, it pulls electrons toward the surface of the silicon, making the transistor easier to turn on. If you trap a negative charge, it pushes electrons away from the surface of the silicon, making the transistor harder to turn on.
You read the stored value by applying a weak voltage to the gate and seeing whether it's enough to turn the transistor on or not.
Every time you change the trapped charge, you have to fight the trapped charge that's already there. Quantum tunneling isn't as reliable as regular through-a-conductor connections, so eventually the encapsulated material can reach a midpoint where it doesn't read or write very well. For today's flash memories, that usually happens after about a hundred thousand read/write cycles.
If you have an older device and have been using it frequently, it's possible that you're starting to rub up against the limits of what the transistors can do.
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