It mounts the USB by itself. If the drive is formated as FAT32 and the phone doesn't detect the device probably there's something with the cable. Download the Xperia Function Tester and test the USB OTG feature. If it does detect the cable in the app, then the usb device that you're trying to connect is faulty.
If your Phone doesn't detect the cable in the app, then the cable is not a "USB OTG".
Test with the app and let me know.
