Does anyone have an idea of what could be the problem or seen this before?

I've never heard of that issue with a USB drive before.

Are you 'hot swapping' the USB drive when this happens? I have had times where the instrument (or a PC) doesn't recognize the drive if you plug it in when the machine is already powered up.

That shouldn't happen - just throwing it out there.

If you aren't using the latest OS version 2.52? If not you might consider updating the OS and seeing if that makes a difference.

https://usa.yamaha.com/products/music_production/synthesizers/modx/downloads.html

Sorry I don't have more to offer.

You may also want to document the USB device (model/manufacturer).  And beyond that, I don't know if there's software that gets a more positive ID on the memory controller used inside the stick/type of memory/etc.  But in the land of counterfeit devices it would somehow be important to know it's legit.  I usually rule this out as much as I can by purchasing from box stores I trust -- but I guess any supplier could be fooled.  Storage devices I try to stay away from Amazon although I admit I haven't kept up if they've improved their track record in the past year or so.

Less important is the legit part of this.  More important to get the model number/manufacturer documented in case others (ideally Yamaha) wants to try to duplicate.

This restart business seems like an over current condition (which shouldn't happen with "thumb" drives aka "sticks" ).  A hard disk could do this since it pulls more current.  Usually there's a simple over current detector on USB ports to gracefully limit over current without "crowbarring" and taking down all of the power rails.

Anyhow at this point I'm most interested in what exactly is being plugged into Montage's port.  And if it's more than one thing (unpowered hub, multiple devices) then all of that would be of interest too.