OK I have temporarily solved the problem. Acknowledgements to those who put this solution on the net.
 
In a terminal, login as superuser:
 
# su
 
Then enter the command:
 
# modprobe usbnet
 
This fixes it.
 
But only for that session. I used to do this sort of thing at the end of the nineties for the soundcard. And I can't remember the commands for making module changes permanent.
 
If I could persuade SUSE to load the module in the setup the problem wouldn't arise.
 
Incidentally, this is the answer for kernel 2.6.x
 
If you have 2.4.x kernel, the command would be:
 
# modprobe CDCEther
 
 
But can someone tell me how to make these changes permanent please.

Once you modprobe the usbnet module, go to Yast and set up the connection. Choose DHCP and to enable the connection on boot. Follow though with the menu windows until the process is finished. Your new connection should be up after the next boot.

Thanks for the input but I guess I'm unclear about precisely what you mean.
 
I have tried both configuring the Motorola (under Network Devices) before a reload and after. Each time after connecting up to the web with the modprobe usbnet command. Even when I had connectivity to the internet, I get the message that there is no driver loaded for this device it may be in the kernel ( or something [sorry should have written it down]).
 
Incidentally, during the reload, I tried looking for anyway I could persuade SUSE to load the usbnet module. There would appear to be none.
 
Anyway, whatever I do, everytime I reboot the computer, I'm not connected again and have to run the command.
 
I feel certain I'm overlooking something obvious and should be setting up one of the other modules in YaST. But there are so many and so little indication of what they are needed for.

Let's see if this will be simple. It's at these times that I wish I had one of these modems to test. Since just loading the module seems to work, try the following;
 
Go to Yast-->System-->Kernel-->MODULES_LOADED_ON_BOOT and add "usbnet" (without the quotations).
 
Bear with me. I am on a new system and don't have my SuSE installation available to me yet.
 
I assume this connection is broadband and not DSL?

Brilliant!
 
That was the solution. Thanks!
 
So to recap:
 
To install a Motorola SB4200 USB Cable Modem using the USB connection, with SUSE Linux 10.0
 
YaST > System > /etc/sysconfig Editor
 
System + Kernel + MODULES_LOADED_ON_BOOT
 
enter:
 
[font:arial]usbnet
 
in the box and click Finish
 
Reboot
 
I understand if you are using an older version with a 2.4.x kernel then the command would be CDCEther, but I can't check this.