i have a Nokia N91 4Gb that crashes when i place the cover (the hard disk doesn't work anymore) and the back becomes very hot and i can hear a sound like zzzzzzzzzzzzzz.
i've upgraded it to the recent firmeware from nokia website but it still stop booting with the cover;
when i remove the cover it's work fine but with only the 16 Mb of memory.
i've tried that tip: *#7370#, power on in safe mode and puting the cover slowly; but and it doesn't work. i waited for all time until the battery made empty and no response from the phone. only hang and lighting screen.