Have you ever flashed the phone at all, even before installing Nitdroid?

You need to have a fully charged battery. If you can't charge it on that phone, get another phone that charges that size battery or buy a cheap separate charger and some cheap extra batteries.

Flashing is easiest if the computer is running Linux, ideally something like Ubuntu.

Some USB ports don't work for flashing, likewise some cables. How long have you had the N900? There can also be a problem with the N900 USB port, but that typically doesn't happen till you've had it for awhile.

Unless you have successfully flashed it in the past, the first assumption is that you don't quite know how to flash it correctly -- that is very common and often seems ominous but it's just a matter of learning. See the Wiki for flashing instructions, but be prepared to be confused till the light dawns.

(I have been interested in Mumbai since I read the book Shantaram.)