Its popular problem on HTC phones. Get the card out of the phone and connect it to adapter (SD adapter). Connect this card to laptop or computer and format it to FAT 32. Then reboot the phone and put the card inside again.
For downloading or saving information on your SD card you must set it in options. In menu you must select all kind of data to be saved into SD card.
Watch this : http
/www.fixedbyvonnie.com/2014/06/install-android-apps-sd-card-default/
You will make it easy
Good luck buddy