do you maybe have a source why its impossible?
If you think about it for a minute, it should be fairly obvious. For a USB device connected to the PC to perform some useful task, it must be programmed to perform that task. Now, the Arduino has a chip on it that is recognized as a USB device, and it can accept commands from the master, and it can provide the correct answers.
However, that chip has NOT been programmed to accept disk-drive enumeration and access commands, and has not been programmed to respond accordingly.
Sure, in theory you could reprogram the chip to do that. If you need to ask here if it has already been done, then it indicates to me that you are not even aware of what it means to program a USB device, which indicates to me that you are not up to the task of doing that.
i cant imagine that id be that first who thinks that that would be very handy.
Well, there are devices that have already been programmed to take a SD card and act like a disk drive (accepting and properly responding to the necessary USB commands). So, the population that needs the Arduino with SD shield to do the same thing is not as large as you might think it is.
"The big thing that is nice about the 8u2 is that advanced users can turn it into a different kind of USB device. For example it can act like a keyboard or mouse. Or a disk driver. Or a MIDI interface, etc. Right now there are no examples of how to do this, but we hope to post some shortly."
Perhaps you missed the highlighted phrase.
But they r wrong?
No, they are right. It is not a trivial task, though, as you seem to think it is.