Given that SD cards have a limited lifetime for writes, using an SD card for a harddrive for Pi may not be the smartest idea in terms of reliability or longevity, so asking how you back up your SD card is a good question. There’s some good answers here.
Specifically on the Mac, find out the disk number of the attached sd card with :
then use the dd command:
dd if=/dev/rdiskx of=/path/to/image bs=1m
where x is the disk number listed from diskutil.