In my previous PC, which had been upgraded from the first Ubuntu MATE through each version to 18.04, plugging in a USB stick would result in the ‘active’ user (i.e. the one whose session appears on the single monitor) mounting it without any fuss.
In this one, with a fresh install of 18.10, typically there are two users logged in and almost inevitably the ‘other’ user gets a popup asking for an admin password to mount the drive. So you have to switch to them via ctrl-alt-Fx, cancel that request, switch back, and enter an admin password for the popup that has just appeared for you.
What did I do to get it to happen as most users would expect on the previous one?