Welcome!
The simple solution is loop&sleep a script filtering <lsusb> to grep certain identifiable info. That way you don't need to mount.
It's a dirty solution, but it works
To unlock just kill the screensaver. Be sure to have the sleep timer a LOT shorter when the USB is removed than when the USB is inserted. When you are away (USB removed) all your machine has to do is fanatically cycle through lsusb looking for your return (USB inserted).
---
I did the same on my machine, but instead of looking for a certain USB device it measures the distance to my mobile phone using Bluetooth. All I need to do to lock my computer is to walk away from it, and to unlock it I just have to come close again 
__________________
Can't tell an OS by it's GUI
|