Reports of fake USB memory sticks and other flash devices have appeared in the press and TV. These are still being sold, as an easy means of making money through fraudulent methods. These devices show the expected capacity, but often use a single smaller flash chip, thereby saving money in the manufacturing process.
When a sector requiring write access beyond the capacity of the single chip is needed, these chips read or write to a position on the same chip, overwriting data, which will be seen as a corruption of the file system. It is often, only when this occurs that the problem becomes apparent.
These devices are often a lot cheaper than the known equivalents, even containing false branding. If the price of a USB thumb drive appears to be too good to be true, it may well be a fake device. Buying a USB thumb drive from a market stall, many of which can be found across the London area, may be a risk, as it is a popular method of selling such storage devices.
It is often the case that by the time the volume is found to be damaged, the data may be severely corrupted or lost. Be very careful when buying cheap USB flash devices, as your data is too important to take any risks.