Not the answer you want, and probably not practically possible, but 10.13.6 is about the furthest MacOS upgrade I'm willing to do for a workstation that does audio and graphics, at the moment. And this works on late model MacBook Airs and Pros, upto 2019 versions. It's a little fiddly to install on 2019 models, default on a fresh install on 2018 and earlier.

Otherwise, sorry, can't help with 10.14.x or 10.15.x

MODX CONNECT doesn't see the MODX either.

Does it give an error message of any kind.

Open MODX CONNECT as a standalone program.
Click on “Setup”
What happens when you attempt to select the “MODX”?

Open MODX CONNECT as a standalone program.
Click on “Setup”
What happens when you attempt to select the “MODX”?

Thanks for your reply! There's no error message, and it's not even an option on the "Setup" panel. There's a field that says 'No Assign" and a dropdown menu next to it that only says "No Assign."

Not the answer you want, and probably not practically possible, but 10.13.6 is about the furthest MacOS upgrade I'm willing to do for a workstation that does audio and graphics, at the moment. And this works on late model MacBook Airs and Pros, upto 2019 versions. It's a little fiddly to install on 2019 models, default on a fresh install on 2018 and earlier.

Otherwise, sorry, can't help with 10.14.x or 10.15.x

Ugh I hope it doesn't come to that. This is my Mainstage only rig, and so I'm sure I must have updated it to keep a VST current. Would be a real a drag if it only ended up having a 4-year life cycle.

Thanks for your reply! There's no error message, and it's not even an option on the "Setup" panel. There's a field that says 'No Assign" and a dropdown menu next to it that only says "No Assign."

This indicates that MIDI is not reaching the computer. You mention the blue USB icon is showing, which indicates the computer is seeing the MODX. This contradiction needs to be resolved. Double check.
Remove all hubs while troubleshooting — just a USB cable from the MODX directly to the computer (nothing in between).

If it connects then we can troubleshoot the devices in between.

Just tried it again and no change - it's going straight into the computer, no hubs or anything else. Definitely the blue USB icon is on - it turns off when I unplug it and on when I plug it back in. I even tried another cable in case I had a bad one!

Can you find the icon for the Yamaha Steinberg USB Driver in your System Preferences?

If so double click the icon to launch the CONTROL PANEL.
It should have three tabs… what do they report?

OK so this was interesting - it wasn't in the System Preferences but an older Yamaha USB MIDI Driver (v1.3.2) was. So I uninstalled both, reinstalled Yamaha Steinberg USB Driver V2.0.5, and now it's back to where it was. The Yamaha Steinberg USB Driver isn't showing up in System Preferences.

I’m sorry but I’m not versed in all the different versions and what the install steps are for the different masOS… make sure when/if you are asked if Yamaha is a trusted vendor you allow it permission to run.

The Yamaha USB MIDI Driver is not a conflict and does not need to be removed. It is the MIDI Driver for all Yamaha MIDI products that do not have a built-in Audio interface.

The MODX doesn’t use it, and it is not a conflict. You may have other products that use this Driver. It has a separate icon, and a completely different list of products.

Not the answer you want, and probably not practically possible, but 10.13.6 is about the furthest MacOS upgrade I'm willing to do for a workstation that does audio and graphics, at the moment. And this works on late model MacBook Airs and Pros, upto 2019 versions. It's a little fiddly to install on 2019 models, default on a fresh install on 2018 and earlier.

Otherwise, sorry, can't help with 10.14.x or 10.15.x

Ugh I hope it doesn't come to that. This is my Mainstage only rig, and so I'm sure I must have updated it to keep a VST current. Would be a real a drag if it only ended up having a 4-year life cycle.

Huge, silent changes, were made to how MacOS manages USB security and connectivity in 10.14 and upwards. There was also a significant change in 10.13, somewhat of a trial run for these later changes, that was slowly fixed through the updates to 10.13.6, although there's still issues with T1 and T2 chip equipped Macs with 10.13.6, they're far less problematic than in newer versions.

Just about all plugins should run, still, in 10.13.6 - about the only thing that won't is new Apple software, and they do that on purpose, because they know best about which version you should be using.

On a positive note, it will only be a matter of time until 10.15 is properly fixed in terms of USB, as there's lots of clamouring about this need, and it will be the long-life legacy version of MacOS for Intel chips, for the life of those products, now that Apple is slowly morphing to M1 etc.

Thanks for your help - unfortunately there seem to be no permissions issues I can discern. I'm reaching out to a product specialist at Yamaha - if he has any brilliant ideas I'll report back!

Whoa whoa this is weird. MIDI works fine when the Yamaha Steinberg USB Driver is uninstalled!! But MODX Connect still isn't seeing it. The driver itself seems to be getting in the way...

Just wrapping this up in case anybody comes across this in the future:

After the umpteenth time uninstalling and reinstalling the Yamaha Steinberg USB driver, MIDI started working properly. No idea what I did. The only thing I did differently was use a different audio interface for a second in between uninstall/reinstalls. Don't know why that would do anything, but there it is.

I wonder if there was anything that needed to be adjusted at the OS level ...

1) Finder
2) Under "Go" menu item (top of screen) - select "Utilities"
3) Click on "Audio MIDI Setup"
4) At top of screen should show "Audio MIDI Setup" with and under "Window" is an option "Show MIDI Studio" - click on that

This doesn't say what to look for or change - only that these are places to look that haven't been mentioned in this thread.

I was looking at the above before when I had connectivity problems with my Macbook - turned out to be an out-of-spec MIDI cable (flat ribbon extracting kind best for USB 1.0 type applications like older printers).