You need to install the plug-in itself on your system drive, and then you can install the library (big file) wherever you want on an external drive, and even move it around, since EW Play uses a browser-based approach to file management. But Play itself wants to live on the system drive.
Does EW Symphonic Gold Library work w/ Kontakt 2 and or 4? Do I have to purchase the Play Engine to play EW stuff?
No you do not need to purchase Kontakt. For two reasons: 1, anything made by soundsonline that uses the kontakt as a host (Symphobia) comes with kontakt player which is free. and 2, EWQLSO (east west quantum leap symphonic orchestra) doesn't use kontakt it uses the Play engine (which is included).