I got it working! After weeks of trying everything. And being super frustrated because both my External HDDs worked perfectly on my GL-M300N-V2s.
To get it working I needed to use external power but mainly:
Updated my GL-B1300 to beta firmware 3.104
Also GL iNET support was absolutely abysmal, I’m not sure if the person I was talking to was incompetent or just apathetic.