I'm assuming the My Books phone home in some way to facilitate file access over the internet. If so, that WD system got cracked. That seems more likely since-- at least for my ISP and I'm guessing most home ISPs-- don't expose individual device ports to the world unless the user sets NAT for that.
Anyone know if NAT configs are required to use the feature that let's you access files from anywhere? If so, someone simply cracked bad security 6 years out of date and scanned for devices.
Given typical IoT security, it might only have been using the default user & password.