Have you tried to reboot the n900?

...otherwise there are plenty of possibilities to share files from the 900.
ssh was mentioned, but there is also possible to use windows filesharing (samba) and even to run a ftp or webserver on the 900 and exchange files that way.
personally i have used samba for ages, and i also guess that is faster than usb. but i havent used usb since i found samba either.