AFAIK there is no method of reconnecting a referenced Photos Lib with its masters files, after say a failure of the hard drive holding the referenced masters. It will offer to reconnect an individual file if you try to edit it, but if you have 60,000 masters on a external you are not going to do them one by one. This means that, even if you have a back up of your referenced masters, the backup can't be reconnected to the Photos Library. I have used referenced Aperture Libraries for many years and from time to time it was necessary to reconnect the masters. Someone please tell me I am wrong. I like Photos as a fully consolidated Library, but use in referenced mode is a non starter without backup capability.