Apple - Cannot erase SD card with Disk Utility (El Capitan)

A better and simpler solution would be using the command-line.

Execute the following command to identify the mount name of SD card following the pattern /dev/diskX, e.g., /dev/disk2

diskutil list 

Say the disk name is /dev/disk2. Now format the card to FAT32 by running the following command:

sudo diskutil eraseDisk FAT32 MYSD MBRFormat /dev/disk2

I had the same exact problem. Format your card using this application: https://www.sdcard.org/downloads/formatter_4/index.html

Worked perfect for me.

Cheers.