Open the Settings app.
Select “System” in the sidebar and click the “Storage” dropdown.
Click “Advanced storage settings”, then “Where new content is saved”
Check which drive is set here for apps download. If it is already set to the drive that contains forza 5 try changing it to c drive applying the settings then restarting your system and changing it back
Sorry for such late response. I tried that and it didn't work, I don't know if that matters but I have Steam version of Forza, so I don't think it matters on what drive new apps are installed