Garmin Express Mac has lost the ability to sync music.

This is very frustrating! Since you've already tried different cables and reset the watch, maybe check for software updates for both the watch and the Express. If that still doesn't work, it might be a good idea to contact support.

I'm having the exact same problem with a 965 and OSX 14.6.1

I have continued to troubleshoot. I am able to manually get to the folders on the watch and add/subtract files that way (I use Commander One). (The MTP connection is completely stable).I have confirmed the disconnects are coming from Garmin Express as I can reproduce them reliability when using G.E. but not via other connection methods. Keeping Express running in the background etc settings make no difference. My working diagnosis is Express is losing it's MTP connection when it reaches out to scan the mac to check folders for transfer content.

Thanks, I didn't know it was using MTP. I used OpenMTP and was able to copy some music over fine.

Same issue here. Please provide how to fix it

Me too!  Forerunner 165 Music Mac OS X Sequoia.  Very frustrating.  It worked for one sync and now it doesn't work anymore.

The same problem

Exact same issue here on a Forerunner 165 Music using a macbook pro on Sequioa 15.0.1