We really like using Etcher for burning SD cards. Works great on Mac OS X 10.9 or later, won't over-write your backup disk drive, and can handle compressed images so you dont need to unzip them!
Download Etcher from https://etcher.io
Open the downloaded disk image and drag the Etcher application to the Applications folder. You can then eject the disk image.
Eject any external storage devices such as USB flash drives and backup hard disks. This makes it easier to identify the SD card. Then insert the SD card into the slot on your computer or into the reader.
The first time you run Etcher you’ll be asked to confirm the download. Click “Open” to continue.
This will launch the Etcher application…
Select the SD card image file by clicking Select Image. You can choose a compressed SD image file such as a .zip or .gz or an uncompressed .img, it’s all good!
Etcher will automatically try to detect the SD drive. If you don’t have an SD card currently inserted, you’ll be prompted to connect one.
Check the disk size to make sure its the right one, that it’s not overwriting your main drive or anything nasty.
Then click Flash! A-ah!
Etcher will work for a few minutes to “burn” the SD image to the card. You’ll see a progress bar as it works. This is about the time you’ll wish you’d splurged on a high-speed card.
Once the SD card is ready, you will see the following:
The card will be unmounted automatically, so you can pull it out now and use it in your Raspberry Pi.
If you find yourself burning a lot of SD cards, you can speed things up by clicking the gear icon at the top-right, then turn off the “Validate write” option. I’ve written hundreds of cards and only had one fail validation.