when I do this the cgminer quits immediately. What should be the usb ?
In order to limit the number of usb devices cgminer will initialize to 1, which is what I think you are trying to do, use:
--usb :1
This doesn't specify which device, but if you only have 1 plugged in it will initialize that one. Then you can plug in more and this instance will not try to see them.