When you install the game, it sets everything up in your user folder (My Documents, My Games, FFXIV... ). In theory, you might be able to change where your default "My Documents" or the entire user profile folder is located within your Windows configuration to point to your other drive and it might work out. May have to re-install for it to pick up the change though, IDK. A junction remap should work too.. but that's a big fat hairy deal to set up without a pre-configured tool.