Hi, I am having an issue when connecting an external WD Passport 256Gb SSD drive to my MacBook Pro retina 2015. The SSD drive is rated at 500MB/s via a USBc connector. I am plugging this into my MacBook Pro USB3.0 input with is rated at around 500MB/s too. The problem is I am only getting 200MB/s transfer rates according to the Black Magic disk speed tool. I am trying to edit video and would like a speed closer to the stated speeds. The drive comes with an adaptor for the cable to go from USBc to the input on the MacBook USB3.0 standard slot. Please can you advise what I might be doing wrong. Thank you in advance, Callum.