System Monitor (or rather, its users) could profit from a feature that auto-adds sensors when they become available to the system. And removes them from the panel (without screaming about “missing sensors”) when they are no longer attached.
E.g, Disks Used Space, as it is now, has to be added manually for the connected disks, and that state remains fixed. A dynamic addition of newly plugged in devices would be nice. Similar to [Group], a menu entry [Auto] could be the interface element to set this.
The optimal solution would be an option to exclude individual sensors, or even classes of sensors. E.g., the disks that are built in to the machine (so only external devices that are dynamically plugged in and out would be auto-added/removed), or exclude USB (but include thunderbolt). Etc.
Of course that feature would not have to be limited to disk sensors.
What do you think?