I have a fully updated retail client of Diablo 3 installed. I have downloaded the public test realm (PTR) installer. When I run this installer, it starts to download and indicates it needs 15GB of free hard drive space. Does the PTR installer download the entire game again, or just the latest (PTR) patch?
The PTR client is completely separate from the standard D3 client, so it will have to be a separate install. Once you've installed it once, any future PTR patches will be downloaded just the same like the normal servers.