Why cant i install software to an external hard drive?

hi there, ive recently purchased a program that intales 50GB of wav files included (toontrack superior drummer if you're interested) and i dont want to install the program to my modest macbook and would like to install it onto my LG 500GB USB hard drive. I'm having trouble installing it to the hard drive, every time i click on my hard drive it says: You Cannot Install Superior Drummer on this volume. why is this?

in disk utility my hard drive is formatted to:

Mac OS Extended

I did this the other day following instructions as i couldnt add any files to the hard drive as it was supposedly formatted wrong.

forgive me, I am really new to Mac so a step by step guide on setting up my hard drive for the program would be great!

thanks

Not really sure to be honest, but this might be worth a read:

The confusing art of installing apps

doug

Also:

can i install an application in an external hard drive (usb) [Archive] - Philippine Macintosh Users Group

Have you actually booted from the external prior to attempting to install the software?