I have a stupid proprietary app that's running on an old windows server 2009 and it ties it's licence key to the hard disk how do I migrate this server thats in questionable health to my current PVE setup?
You will have to find out what characteristics of the disk it uses for this. Could be lots of things, the disk serial number, the filesystem UUID, any number of things. Or you could contact your vendor and ask for support to move the app to a new hard drive.