I have a WD Mybook 3tb drive setup as my usb storage, and it is very unreliable. At times it works fine, but is unusble a large majority of the time. As in, it can't access a single file on the drive. It doesn't seem to be related to access either, as it sometimes happens under heavy usage, other times accessing a file at a time. The drive had no problems when plugged into a computer.
All of my settings are default, no secure access, no media server or ftp server, no media prioritization.