If you previously followed this guide for Cataclysm, remove all of the links previously created, delete all WoW files from your SSD and rerun your launcher to install the missing pieces before following these instructions.
UPDATE: The guide has been updated for MoP. As with all expansions, there's obviously going to be more space required for the install, and with MoP's significantly higher amount of textures and assets compared to previous expansions, this is no different. It seems they have also fooled around with MPQ groupings as usual, but the overall purpose of this hasn't changed.
At first glance after I (and after you have) installed the 5.0 client, you'll notice that the total folder should weigh in around 20GB. So you'll think, "Gee Glo, what's the point of doing all of this for a measely 5GB?".
Well, the point is:
1. Your sound cache will start to build up gradually over time as it has previously and drastically inflate your install folder size
2. Keeping your logs, addons, and everything else off of the SSD will prevent virtually all writes to your SSD from WoW. If one knows about SSD longevity and the factors contributing to it, this is clearly an awesome thing. WoW only -reads- info from MPQ files, it doesn't write. The only time you'll EVER write to your SSD with WoW is when you patch.
Back to the guide!
First, a background on how WoW works with its own data files:
WoW stores all of it's resources in large files called MPQs. These files contain sounds, textures, and base polygons, among other things. Every time you load into a zone, the client polls the corresponding MPQ file for the needed resource.
So how do we reduce that amount of data? By putting resource files that don't need SSD performance on our regular old mechanical HDD. Sound resources, mainly. Sound (and mostly the sound cache) take up almost 9GB of WoW's installation size, and since soundtracks are only one sequential read, having it on a mechanical HDD will virtually have no negative performance decrease as a result. The second component of WoW's sound are combat noises, spell noises, and the such. Those a very few in number, and again, will have no impact on your load times nor overall performance.
How do you split up the installation? We're going to use a nifty little feature in Windows called "Symbolic Links". It's simple!
Download this program first to make using symbolic links easy, without the need for the command line:
Pick x86 if you are using Windows 32bit, and x64 if you're using 64bit. Make sure you download and install BOTH files so you have the necessary DLLs to run the program.
After you're done with that, just follow these very simple steps:
0. If you previously followed this guide for Cataclysm, remove all of the links previously created, delete all WoW files from your SSD and rerun your launcher to install the missing pieces before following these instructions.
1. Move (or install) WoW to your mechanical HDD in its full form.
2. Delete your WoW installation from your SSD.
3. Create a folder anywhere on your SSD, it doesn't matter where it's located. Name it "WoWSSD" for easy reference.
4. Navigate to your WoW folder, then enter the Data folder within and select the following files that are pictured in this screenshot.
5. Cut and paste those files into the "WoWSSD" folder you made on your SSD. Do not copy and paste them, they need to be completely removed (or renamed) from your mechanical HDD's WoW installation folder.
6. Navigate to the "WoWSSD" folder, and highlight all the files within it. Right click and select "Pick Link Source".
7. Navigate to your mechanical HDD's WoW installation folder, then navigate to the "Data" folder within it (the same place you copied the data files from previously).
8. Right click inside the folder and select "Drop As", then select "Symbolic Link".
9. Fire up WoW and enjoy your extra SSD space!
(if you opted to move your installation, the shortcuts on your start menu and desktop will need to be updated with the correct path!)