Windows 8 installs applications to your C:\ drive by default, but you may want to change where Windows 8 stores these apps. For example, you could install them to an SD card or secondary hard drive.
This works on both Windows 8 and Windows RT. It’s useful whether you are using a Microsoft Surface, another Windows 8 tablet, or just a computer with a small SSD and a larger secondary drive.
Prepare the New Drive
Before we begin, we must prepare the new install location. First, open a File Explorer window. Right-click the drive where you want to install Windows 8 apps – whether it’s an SD card, hard drive, or whatever else – and select Properties.
Ensure that the drive is formatted as NTFS.
If the drive isn’t formatted as NTFS, you will need to format the drive as NTFS to continue. (Many SD cards are formatted with the FAT file system instead.)
Back up any important data from the drive before continuing – formatting will delete all data on the drive. Right-click the drive and select Format.
Choose the NTFS file system and click Start.
You will want to create a folder for the apps on the drive. You can name it whatever you like, such as WindowsApps or Windows8Apps.
Change the Registry Setting
You will now need to open the registry editor. Press the Windows key, type regedit at the Start screen, and press Enter.
Navigate to the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Appx key.
Select the Appx key, click Edit, and click Permissions. (Ensure the Appx key is selected or you’ll modify permissions for another key instead!)
Click the Advanced button.
Click the Change link next to TrustedInstaller.
Type Administrators into the box, click Check Names, and click OK. This will give all administrators on the computer ownership of the registry key.
Enable the Replace owner on subcontainers and objects checkbox and click OK.
Select Administrators in the Permissions for AppX window and click the Allow option next to Full Control. Click OK and you will now be able to edit the appropriate registry setting.
Double-click the PackageRoot value in the right pane and enter the location of the folder you created earlier. That’s E:\Windows8Apps\ in the case of our example.
Reboot your computer. Your changes won’t take effect until you reboot.
Move Old Apps to the New Location [Optional]
This will only affect apps you install in the future. Previously installed apps will remain at the old location. To move these apps to the new location, you can uninstall them from your computer and then reinstall them from the Windows Store.
Thanks to tamarasu on the XDA Developers forum for first discovering this geeky trick!