I must use rescueOS by NIN101. When I first time connected phone to USB (running rescueOS on computer) it started as is described in rescueOS documentation (without pressing u key on N900). I started to charge battery, but it could not be stopped in any way so I must have remove battery. Since then when I connect my N900 to computer I must press u key. In other way N900 is not booting with rescue OS. Does somebody know what is the reason of that situation?