Really? This doesn't work for me. But maybe it was my phone, or maybe it only does that when connected to a Windows machine (I mostly run Linux and am not about to dig out a Windows machine just to check, at least not right now).

If you have installed the rootsh package, you can open the terminal, get root, then install any package you like using apt-get, for example you'd install the ssh server with the command:
apt-get install openssh-server

With the ssh server installed you can use it transfer files. One possible problem with this is that I can't figure out an easy way to make the n900 connect to a wireless access point from the command line.

With the terminal available you can copy files from the main storage to a micro sd card. If you have enough space on your microsd card you can copy (almost) everything with the command:
cp -r /home/user/MyDocs /media/mmc1/

I put the almost in brackets there because there are some configuration files (and maybe high scores for games) stored in /home/user, so you might want to copy all that as well if you think there'll be anything you care about saving in it.