This post describes a solution for the problem when a magic trackpad connect connect to OS X after un-pairing.

After having multiple issues with the trackpad, I un-paired the device, hoping when I re-paired it, the erratic behavior would go away. Unfortunately, the computer was unable to discover the Magic Trackpad. The solution rests within the Bluetooth Explorer within the Hardware IO Tools for Xcode. This is a free download if you are registered for Apple’s Developer Program (also free).

I used the Bluetooth Explorer and tried two different approaches, not sure if both or just the latter solved the problem.

  1. Remove the existing entry from the cache by selecting Device Cache Explorer from the Tools Menu. I chose to Delete All as I did not have many Bluetooth devices connected.
    Bluetooth Device Cache
  2. Next, select Modify Software & Device Configuration from the Tools Menu. I selected the following settings, shown in the image below, clicked Perform actions, and then rebooted the computer afterwards.Modify Software Configuration

After reboot, the Magic Trackpad was able to pair with the computer and has been performing well.