My Cloud becomes inaccessible when a USB drive is connected

[dubie]

You can kill the media scanning by running some commands by SSH into the MyCloud. Check the following post:

http://community.wd.com/t5/WD-My-Cloud/My-Solution-to-My-Cloud-is-disconnecting-from-the-Network-and/td-p/680091

I fully understand your reluctance to downgrade the firmware. However, if you take some time to browse through the various posts in this forum you will see evidence that the most recent firmware updates from WD have caused more problems than they have solved. So, many of us have found that a simple solution is to downgrade. You can study the method here:

http://community.wd.com/t5/WD-My-Cloud/Easy-method-to-downgrade-to-previous-version-of-the-firmware/td-p/773073

Unfortunately the only way to get a functioning device seems to involved hacking into the device in one way or another.

If you prefer not to do that I can only suggest you contact WD support directly. Good luck!