What I figure out from internet searching and personal tries is that the unifying usb receiver does not come with normally purchased products. You purchase it separately and can pair with a total of at most 6 devices. I have a few Logitech mice some of which has no usb receivers and none can use the others' receiver. I will now purchase a unifying usb receiver to be able to use them.
A mouse can be paired with a single receiver at a time. So, to restore the connection to the original receiver (if you have), you need the Logitech Connection Utility Software (LCUS).
I have just figured out that LCUS also allows you to pair a Logitech mouse with a usb receiver, e.g., my blue mouse has no left button but has a working usb receiver and I lost the receiver of my fully functioning red mouse, I could match the blue receiver with red mouse with this software.
Links
- http://support.logitech.com/en_us/software/unifying
- http://support.logitech.com/en_us/software/logitech-connection-utility-software