If you installed the network-over-USB driver, which seems to be a prerequisite for qtopia, then you have flashed an incompatible kernel part to your firmware. An F200 comes with 4.0.0 or higher, while the driver is based on a modded 1.4.0 kernel.
The best way to restore this is to "update" your firmware to 4.0.0, using the full update you can find at
http://archive.gp2x.de/cgi-bin/cfiles.cgi?0,0,0,0,42,2423
After that, your SD should work again. I'm afraid qtopia is incompatible with the F200, since the F200 is incompatible with any firmware version lower than 4.0.0, unless qtopia also runs without the network-over-USB driver. Besides, don't expect qtopia to support the touchscreen, unless someone compiles a new version.
How can I flash my GP2x without the sd card slot? It doesn't read the sd card at all.