Welcome to Android Central! When you connect via USB, swipe down the notification panel and tap the USB notification. Make sure it's set to MTP. On the computer, open Windows Explorer and look for the device in the list. It won't be listed as an actual drive--that only works for older phones whose internal storage is divided into a smaller app storage area and an "internal SD"--it's the latter that could be mounted as a drive when connected to the computer (and therefore unmounted from the phone).