If you need to reinstall a drum library, log into your Native Instruments account in Native Access. You will be able to find your library under the "not installed" tab or via the search feature.
You do not need to enter your serial again. Entering the serial after your library has already been registered will result in an error message.
If you are not seeing your library, make sure that you are logged into the correct Native Instruments account. If you cannot remember which Native Instruments account you used to register your library, please reach out to us privately for assistance.