It's possible, but it does require some work. Microsoft doesn't make this easy. You can use directory symbolic links (symlinks) to put the files on one drive (say, D: ) but have them accessible via another (say, C: ). This avoids breaking all the install paths, etc. and ensures new installations will go to the correct disk. Please note that even the biggest Metro apps are pretty small compared to the typical Steam game. You'll save a lot more space moving your Steam library over to the other drive. It's also a lot easier; you can either use the built-in Steam feature to put games in different library locations, or you can use symlinks (similar to as below, but with a whole lot less "Takeown" and similar). However, if you really want to move the WindowsApps folder, try the following steps. Be aware that in cases of mistyped commands, failure to follow instructions correctly, gremlins, or plain bad luck, it's entirely possible that this will mess something up.
You'll need an Administrator command prompt (meaning you need Admin access) to do this.
These steps assume that your C:\ drive is your SSD, and that Windows is currently installed there, and D:\ is your large data drive, where you want to move the app folder.
You may adjust the paths, especially the destination, if you want; these instructions put the "WindowsApps folder on the root of the D: drive.
First, make sure you aren't running any "Metro" apps. It may help to do these steps immediately after rebooting.
Open a Command Prompt as Administrator (you can do this by right-clicking the Start button and selecting "Command Prompt (Admin)" from the menu).
Take ownership of the WindowsApps directory:
takeown /F "C:\Program Files\WindowsApps" /A /R
Make a copy of the "C:\Program FilesWindowsApps" folder onto the new drive:
robocopy "C:\Program Files\WindowsApps" "D:\WindowsApps" /E /COPYALL /DCOPY:DAT
Make sure that the copy succeeded (no Skipped or FAILED, etc. in the summary).
Delete the original WindowsApps folder:
rmdir /S "C:\Program Files\WindowsApps"
Create the symlink:
mklink /D "C:\Program Files\WindowsApps" "D:\WindowsApps"
At this point, you should be done. Try running a Metro app to verify that it worked.