Can anyone explain Underpotential Disposition to me in simple terms?
No, but you did choose the relatively unknown initialism "UPD" to describe it, in preference to the many much more popular acronyms or initialisms in use for these devices. When you have to resort to creating a web page on dropbox, and hotlink to it, purely to explain your use of the term, then you really should consider whether your true motive is clarity, or just plain stubbornness in the face of adverse comment to your poor choice of abbreviation.I didn't choose "pen drive" any more than I chose "thumb drive". This has already been discussed to exhaustion and is closed as far as I am concerned. What you choose to call it is up to you.