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