To install a Windows 8 app on an SD card, make sure that the card uses the NTFS file system format. If the card isn't in the correct format, you need to reformat it to the NTFS format.
Edit the Windows registry to give yourself permission to change the location of the installed Windows 8 apps. To do this, you open the Registry Editor, find the registry entry for HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Appx, and right click on it to change the permission. After that, double click the value name "PackageRoot" of this entry and change the value data to the SD card. Click the OK button to save the change to the registry and restart your device to apply the changes.