Backup iPhone to external drive on Mac
Ok I FINALLY got this to work!! The alias solution mentioned above actually really helped. I tweaked the process slightly and looked at other references to get this to work.  I'm using a WD My Cloud NAS drive to store iOS backups. Heres' what I did, step by step: 


Create a new folder on the WD external drive and name it 'Backup'
Opened the Terminal app and typed in the following line: 


ln -s /Volumes//Documents/Backup ~/Library/Application\ Support/MobileSync/Backup

*Be sure to use the name of your external hard drive where I've used the WD My Cloud drive.

What this ended up doing was to create a brand new Alias folder of the 'Backup' folder that I created on the WD MY cloud drive (alias folders have the arrow on them meaning they point elsewhere. In my case to the WD drive). It created this new Alias folder within the original 'Backup' folder on my computer. This is the new folder that I want iTunes to back up my iOS devices to, as it sends all the data to my external hard drive without taking up space on my computer. A few more steps to get this finalized


Now since the Alias folder resides within the original 'Backup' folder on my computer, we need to remove it from there and bring it up one level to be at the same level as the original backup folder. Before moving it, we need to rename the original 'Backup' folder to something like 'Backup_Old'. 
So this is what things look like now: Library > Application Support > MobileSync > Backup_Old > Backup (this is the Alias folder with the arrow).  Now all we have to do is move the 'Backup' folder up to the same level and we're good to go!


Test it out by backing up your device through iTunes and you'll notice that the Alias folder on your computer takes up like 4kb while, the contents inside actually take up a whole lot more. Also to verify, check the folder on your external hard drive and it should be fairly large. 

I used this site to help with the terminal syntax part: http://support.digidna.net/hc/en-us/articles/203504123-Storing-your-iPhone-Backups-on-an-Alternate-Location