When I copy files using Nautilus it tells me that it copy the file at 50Mo/s. I know that it is impossible for my USB 2.0 drive. It stuck then when there is only 2~3Mo remaining. How to solve this problem?
The copying is likely to be cached and/or buffered (in memory) which is why you see the high numbers.
To see "real" speed you would need to remove any caches and buffers, but the effect of it would probably be a slower than normal copy operation. To achieve this you need to mess with file system internals and system memory usage related to that.
Flash memory needs to have a certain amount of data to write at a time for optimal/fastest possible speed.
Copy to USB memory stick really slow?