Good luck with the move.
Also maybe you already know, but in case you don't; To verify that all files / connections is closed and everyone properly disconnected (except for you as admin), on the old server you can go to "computer management", "Shared Folders", "Sessions" or "Opened files", to check all remote connections.
Like I said before too, you might also want to stop/disable any services that might use those shares till your drive is properly connected on Proxmox again.
