I did a full rsync of my N900 OS partition "/" but i dont think you can rsync it back when you totally screwed up your N900 because you prob need pc-connectivity installed on the phone to use ssh or other types to connect to your root.

When you plug it in your usb port you only see the Flash Card and the 27Gb part of the 32GB HDD witch can be used for storage and installs, but not the / (OS part). Prob a hidden partition that can be accessed by using the pc-connectivity apps and simply adding network location "ssh" type and entering your phone IP (with pc-connectivity that is 192.168.2.15) and you CAN access the / "Root" of your device.