Basically, all I had to do is press the reset button. It was not showed in any manuals or so had hard time searching it. Now, it was located:

Just had to press it using a needle and voilĂ . The problem is solved.
Merry Christmas, by the way.
I have the same problem as mentioned earlier about the iRiver T30 on ubuntu. After reading through some threads on the misticriver-forum, I found out that the US and European version can't be used on linux because it's firmware is based on mtp (microsoft "secure" firmware) which ties it as mentioned in an earlier post, to windows media player. Further more iRiver isn't planning to release any ums-firm/software for the t-models, so all in all it seems that we just have to wait and see what iRiver does or someone manages to find a workaround.