Error apply Hard Drive changes | Failed to add device 'Synthetic Disk Drive' | Hyper-V

So today I was adding a new disk to a clustered virtual machine running in a Hyper-V environment.
When I clicked 'apply' to save the changes I'd made, I received the following error message:

The device was being saved from one of the Virtual Machine Hosts, to a SAN where all the HDDs were located.

Luckily, this was a nice and easy fix.  When you're going into the drive settings, check on the top-left corner where it shows you the iSCSI Controller / IDE Controller.

Simply change this to one of the other settings, and click 'apply', to see whether it will let you save it.  In my case, it was set to IDE Controller 0, and I needed to change it to SCSI Controller.

Once you've changed this, you should be good to go!