Just a regular 2.5″ 250GB wd passport drive. Tried it earlier, saw that drive connected but that the drive didn’t power-up enough.
So, thought that it would work if I could power the drive from outside the phone. So, got an usb otg cable with usb power. And, it worked JUST fine. Just make sure that the drive is FAT32 formatted, or it will NOT work.
This with a newer passport drive will most likely not work since they’re NTFS formatted for windows , and I’m not sure if the wd passport tool will work for them
I connected a regular htc usb charger to the extra usb power port with a regular usb extension cable.
Edit: If you want to make it a fully mobile solution, you can power it with a battery bank. Like the htc double charge (HTC BB G600), just tried it and it works.
Got a cable similar to this one on ebay