Are there any other usb devices connected to your PC? If you have, you could try the following:
PC:
1. disconnect all usb devices
2. power off PC
3. boot PC
Cybook:
4. remove SD card from Cybook
5. reboot Cybook
6. connect Cybook to PC, without any other usb devices attached
The reason for this approach is, that there are some linux drivers for usb devices which are a bit clumsy (even on my favourite Gentoo system)
