Nope you don’t have to purchase anything. If you have registered it & have 1 authorization left (you get 2), you can just install EZDrummer 1. I did it on my High Siera system (10.13.5) & it works fine. Keep in mind that Toontrack no longer supports it, so you probably can’t open a ticket if you need help. That said I’ve owned EXD1 since it first came out & I’ve never had ANY issues with it. For me it’s the easiest to use, best sounding & most rock solid, straight forward app ever, not to mention easist drummer to get along with, I’ve ever played with.
You should probably download & use the Product Manager software (available inth Products section on this site) to make sure you’re downloading the correct version & get all of the correct updates. When EZD1 came out there were very few 64bit programs & as I recall EZD installed a 32bit program & it had to be updated to 64bit. OS X used to let 32bit VST & AU install, but as of a few versions ago, 32 bit stuff no longer works. It installs, but it doesn’t work in Logic or Mainstage. I am not certain if 32bit instruments will work in 10.11, but 64bit is the way to go anyway. Install with the PM & then just check in the update area & have it only install the 64bit EXD updates & you’ll be good to go.
