This is usually an indication that some critical data on the drive is corrupted beyond repair. Although this can indicate a defective drive, this can happen as a result being disconnected from the computer while data is still being written to the drive. After such an error condition, on rare occasions we have seen successful data recovery. Recovery is rare and clearly depends on the exact nature of the corruption.
The way to avoid this problem is by taking extra care when disconnecting the drive from the computer. Windows may use a performance enhancing technique called "lazy writes" where data being written to the drive is cached and not written immediately to give the processor time to perform other tasks. Because of this, it's a good idea to allow an extra 10 seconds after Windows says the transfer is complete before unplugging the USB Flash drive. Also, when using the drive on Windows, it's a very good idea to use the Windows "Safely Remove Hardware" tool before unplugging the drive. The "Safely Remove Hardware" tool can be found on the taskbar near the clock.