With Loliipop, you can write to the SD card if the app supports it. The app has to be specifically re-written to allow writing to SD card. On my Samsung Galaxy Note 4, I use an photo album app called Quick Pic. Previously when using the app in KitKat 4.4.2 on both the Galaxy Note 4 and Galaxy S4, I cannot write to the SD card from within the app - so even simply things like rotating a photo was not possible. However, after my Galaxy Note 4 was updated to Lollipop this year and the Quick Pic app was updated, I found that I can write to the SD card from within the Quick Pic app now. Actions like rotation, cropping, moving pictures from one folder to another within the app are all now possible.
Hope this helps... so you might want to update the MiPad to Lollipop if it is available now and also check that the app you are using is also updated for Lollipop and that might allow you to write to the SD card from within the app.
Note that some apps are not updated even on Lollipop. One of my apps - OfficeSuite 8 which I use on a Sony Tablet Z still cannot write to the SD card even though my Tablet Z was also updated to Lollipop this year and the OfficeSuite 8 received an update. Just too bad lor... I have to find a work around... like save the Word file on system memory then transfer to the SD card using the File Manager from Sony lor...