Can't even load the game

[Kittra]

A little more information and experimentation would go a long way in helping others understand your problem Psiphre.

You said it crashes when trying to launch from the desktop shortcut? Have you tried going to the installation folder and launching the game directly with ffxivboot or tried to configure the game differently with ffxivconfig?

If I understand correctly, you've tried running it on two different operating systems on the "same" computer but installed on different hard drives. Is the game also installed on both hard drives or just one of the two?
If it's just one, it could be a faulty installation and a reinstall might be necessary.

Have you been able to launch the game before a recent patch(es) or is this your first time attempting to load the game on your system?

Have you launched task manager after attempting to launch the game and looked in the "Processes" tab for ffxivboot.exe *32 or ffxivlogin.exe *32?

I have experienced a problem in the past which is similar to yours in that the updater would load for a split second, disappear, and when the login page "should" appear nothing was there...

By opening up task manager I discovered that--even though it was not showing up in the "Applications" tab--ffxivlogin.exe *32 "was" running, it just wasn't showing up on my screen. By accident I discovered the solution was to do Alt+Tab. For some reason FFXIV will sometimes "minimize" itself on launch and not show up in the taskbar (the bar your START button for windows is located on) but once you Alt+Tab you can select it and it will pop back into existence.

Stupid question: Have you reset your system after the problem pops up?

Lastly, what are your system specs?