Having issues unmounting partition for debian install

I have a 50gb partition set up on my mac for installing debian, MS-DOS (FAT) format. I blessed it and am now unmounting it in order to copy the img file over. I'm using: $ diskutil unmountDisk /dev/disk0s5 to unmount and am planning to then use: $ sudo dd if=debian7.8.0-amd64-CD-1.img of=/dev/disk0s5 bs=1m to copy the img file to the partition.

The problem is every time I try to execute the unmount on Term, it gives me Unmount of disk0 failed: at least one volume could not be unmounted. I've verified this is the right disk using diskutil list. Any ideas what is wrong?

Sorry for the simple question, I'm just starting with Linux.