Can I use a SD card to USB adapter to directly txfr photos from my SD card onto the external drive?
I want to be able to directly save photos I take from the camera's SD card to this drive while out in the field (avoid having to carry laptop with me if possible).
Sorry no you'll need power and some sort of computer interface (PC). They do make specialty drives that do just what you are asking for, but are now are near this price range.