diskutil list
sudo diskutil unmount /dev/disk4

# 假如出現disk4 was already unmounted or it has a partitioning scheme so use "diskutil unmountDisk" instead
sudo diskutil unmountDisk /dev/disk4

# 加上r會快很多
sudo dd if=2019-09-26-raspbian-buster-lite.img  of=/dev/rdisk4 bs=1m

# 裝完SD卡之後再unmount
sudo diskutil unmountDisk /dev/disk4