What actually happens with an USB flash drive when it dies?

Solution 1:

Flash memory cells use floating gate transistors to store each bit. The "floating" part of semiconductor is surrounded by an oxide and acts like a reservoir for electrical charge. Electrons can be "tunnelled" into or out of the floating gate (through the oxide), by applying the appropriate voltage. Otherwise the charge is stuck in the floating gate, even when the power supply is removed. However, over time (after many program-erase cycles) this oxide can wear down, and the cell loses its ability to store a bit.

The controller of a flash storage device might anticipate this problem and can replace worn areas of memory with spare area. This, along with a wear-levelling strategy, should prolong the life of the drive.

Solution 2:

From eHow.com:

Causes of Flash Drive Failure:

Mistreatment

A flash drive can be mistreated to the point where it fails. For example, if you plug your flash drive in and remove it incorrectly, then you may be putting an unnecessary amount of pressure on it, causing weakness in the solder joints. Or if you bump the flash drive while it is inserted in the computer, you can bend it at the solder joints. This can crack the solder joints, severing the connection and causing the flash drive to fail. Flash drives are relatively fragile, especially the connection between the USB connector and the flash circuit board.

Computer Failure and Power Surge

If a computer fails while a flash drive is plugged in to a USB port, the flash drive may fail as well. Or if a power surge occurs while a flash drive is plugged into a computer, this may also cause the flash drive to fail. While many flash drives can survive these occurrences, it is important that you understand that such an event could potentially cause harm to an attached flash drive and the data stored on it.

Virus Attacks

If a host computer is attacked by a virus while a flash drive is connected, then the flash drive will also be subject to attack and may become damaged. The data stored on the flash drive, then, may become corrupted or otherwise damaged or erased in the process.

Read and Write Cycle Loss

Flash drives all fail over time because there are only a finite number of read and write cycles available on them. So over time, a flash memory drive will simply wear out and need to be replaced. The memory chip inside the flash drive will fail through repeated use, but very few make it this far because they are susceptible to so many other forms of damage.