Raspberry Pi and Arduino issue
 

When I plug the arduino uno into the raspberry Pi the mouse stops working. Anyone know why? What is even more interesting is the light on the arduino pulses and the light on the optical mouse pulses - at the same rate.??

Check your power supply.

My guess is that you've a power supply that can't supply enough current for both devices.

A common thing for people to use is whatever old mobile phone charger they have lying around, some of these provide as little as 500ma compared to some of the newer power supplies that can provide up to 2A.