My galaxy note 3 works just fine without an external power source. I just plug it in the otg then done. I use ES file explorer usually but it also works on the normal files app

Ur hard drive's cable should have 3 USB connectors like Y shape for one to go on another power supply and one on the HD and the last one on the otg which u'll connect to your phone. U'll need another power supply cuz ur device might not be capable of supplying the power ur HD needs

You will need an app that can read NTFS File systems

It depends on what type of phone you have. Each phone has it's limits of the size of the external drive it can read.