I had similar problems with my Motorola SB4200 Cable modem under Mandrake 10.1. Initial research pointed me to using the CDCEther driver but this didn't work. However, I discovered that for the later kernel versions the USBNet driver should be used. This is how I got my modem up and running.
1. Go to the Mandrake Linux Control Centre and select the Network & Internet tab
2. Select the New Connection icon
3. Select Cable Connection and click Next
4. Follow the default selections and when prompted to select a driver use USBNet.
I also found that despite selecting the 'Start at Boot' option, I had to type 'modprobe usbnet' as root after each reboot. To resolve this I edited /etc/modprobe.conf to add the line 'alias eth0 usbnet' at the end. For more on this see thread http://www.linuxquestions.org/questi...82#post1314382
Hope this helps