Tested on MacOs Sierra 10.12.6 and Virtualbox 5.2.18 (Oct 2018) Firstly, ignore advice about having to disable System Protection Integrity, it’s erroneous. csrutil status System Integrity Protection status: enabled. Ok, here are the steps: Insert an sdcard into the slot - MAKE VERY SURE YOU HAVEN’T FLICKED THE WRITE PROTECT SWITCH ON THE CARD - it’s very easy to accidentally move it on insertion if it’s loose. Carefully use a pin to push it back to READ/WRITE mode.