Cannot add music to iphone from mac

This is now driving me insane. I have stored my music on an additional drive for years, should I wish to change music on my phone (Iphone 14), I hook up the drive and go. Tried to do this a couple of days ago, the sync failed and almost all my music disappeared from my phone. Been trying to rectify ever since and it just refuses to do it.

I've bought a new drive in case the old one was corrupted but that hasn't helped. Both my device and my mac (Macbook Air 2020) are signed in to my account on Apple. Have hard reset both of them. Still nothing. Tried moving through finder and drop and dragging in Music. Tried wireless and through USB. Am having no other problems with either device.

Completely out of ideas. Can anyone help?