Apparently, this question has been asked long ago and a response was generated long ago, but the solution presented does not work. Perhaps it is time to open a new thread in hopes that someone new from Microsoft will read it and give a new response.
I just purchased a Windows 8.1 tablet. I am very excited to get in and start using it, but the 32GB hard drive is eaten up very quickly, so I would like to install Windows Apps on my SD card. In researching this, I found that it was an option with Windows 8, but the functionality was removed for Windows 8.1. Nonetheless, I followed the steps laid out back in March by a Microsoft Support Engineer, and was unsuccessful, just as everyone else claimed in the following posts.
If there is a way to install Apps to an SD card, please inform. If not, please make a patch to Windows 8.1 to allow this.