If you've lost or damaged the USB receiver for your mouse, you'll need to identify which receiver you need before purchasing a replacement.
If you see the Unifying logo on the bottom of your mouse, you can purchase a replacement from the M310 Parts Store.
NOTE: There is no store for the M305. The Unifying receiver in the M310 store will work with your Unifying-ready M305 mouse.
The Unifying logo looks like this:
If you don't see the Unifying logo on the bottom of your M305 mouse, contact Customer Care for assistance (see answer 12127).