Recently and for no obvious reason, my Eee PC (still running the original Xandros install) started refusing to read USB drives, popping up the not very helpful message 'You do not have enough permissions to read '. At first I feared the machine was on a rapid path to disintegration, but a little Google-ing and a helpful hint from the EeeUser Forum got me back on track. All that's needed is to bring up a terminal window (using Control-Alt-T) and then typing the following commands;
sudo -i mkdir /media
The accepted consensus seems to be that this problem occurs if you don't 'safely' eject drives by clicking on the USB icon -- but it's a fast enough fix (and uncommon enough problem) that I'll probably keep being lazy and pulling them out just before shutdown anyway.