I had the same problem, your rootfs on N900 got mounted instead of your sdcard, so it installed NITdroid there and the maemo OS already takes up alot of space + apps. Thats why you had 0mb's left there and you went out of space. You must never reboot your N900 when in 'trouble' it doesn't work like windows that some problems get fixed with a reboot. Just come back here in the forums and ask what to do if ever in a situation like this again

Theres was a simple solution for your problem, if remember correct:

cd /and 'enter'
rm -r * 'enter'
And then start over with the mounting and extracting.
(somebody correct me if i'm wrong)

Sounds like a reflash

e: I correct myself, the correct code is:
cd /and 'enter'
rm -rf * 'enter'