Have you tried to run the application ‘Console’ under the directory Applications/Utilities ?
Start the app, then when the window comes up, look top right, and you should see a search field. Type “diskm” in this field, and then hit return.
You can save this type of search in console to quickly return to specific searches by tapping the save icon. Others might be wifi, network, mountd, Bluetooth, …..
There are presets listed on top left, such as , All messages, Errors, and syslog
Now you need to start viewing data being sent to console.
Do this by hitting the start icon in the console window. If it doesn’t start putting out messages, make sure the “now” button is selected as well.
So whats this going to do for you? It might help why at the last point the pop up window with error comes up, when it does happen.
At this point look in console, you could copy and paste the diskm output before and after the pop up comes up.
You might get lucky and it state a clue to the drive, app hangs, disk not found, etc etc…
That data and a call to Apple support over the phone might take you to the next step.