In case it was not clear: connecting a mobile phone to a computer and using it as a usb storage devices is simply not possible, doubly so if you are looking for platform independence (windows/Mac/Linux). At the very least you will have to use external specialized apps, or jailbreak your device, or a combination of both, if it's at all possible.

This was a basic feature that all phones used to have. It was quietly removed, I suspect in an effort to push users towards iTunes/Google Play Music, both of which are dead now.