You have two options for scripting drive unmounting: 1) the terminal command unmount and 2) the AppleScript command eject.

tap.applescript is the only object that I know of to run AppleScripts from Max, so if you don’t have tap.tools, maybe that won’t work.

The other option is to use shell or aka.shell to run terminal commands. You could use those objects to run the unmount command or to run an AppleScript via the osascript command. Haven’t tested doing this particular shell stuff via Max, but should work in theory.