Unauthorised disk - No issue as input is effectively disabled.
Audio CD - Will probably stop playing music (pause?)
VCD? No idea. I don't have a SCPH-5903 VideoCD
PlayStation Disc - We can remove Memory Card after a timeout if the MC has not been read - This would disable FreePSXBoot from running if the user had an Audio CD in the drive
PlayStation Disc with patched BIOS - We can detect disk by GameID
Device in parallel port - Mayhem? Can we detect this?
Touched on in https://github.com/dangiu/PicoMemcard/issues/8