i don't know why but my N900 seems not be able to boot without a usb connection. I have flashed new and old firmwares, and emmc images, changed kernels, but nothing seems to work.
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Reboot-Loop
Nokia-Logo without Backlight
three dots
reboot
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I can start my n900 (boot into maemo) if i connect a usb cabel, so my first guess was, that then900 cant get enougth power out of the battery, so i replaced it, but it didnt help at all.
I have an external battery-charger to, so there shouldnt be a problem with the charge of the battery, or anything else.
Thats an odd one, i have not heard of that before.
I had the exact opposite my phone would not boot with usb inserted (mini usb or wall charger) so in theory i couldn't charge the phone but it booted with a freshly charged battery, later it was found to be a faulty mini usb socket on the phone, in the end i had to swap my N900 for another one.
I had the exact same problem, tried everything, nothing worked, then I returned to the seller and got a new device, and it works perfecly, unfortunately you will have to do the same, some devices came with manufactory deffect, and there is nothing you can do about it other than return it asap.