What USB External Hard Drives are Compatible

I tried to add an external USB hard drive to my My Cloud today. It is a Seagate USB 3.0, 3TB. All it seems to do is to make the power button blink red. The support material keeps referring to adding a “compatible external hard drive”, but never says what a compatible hard drive is. Is there a list of them?

Is there a list? Don’t know. However, the problem for some, including myself, with external USB drives only started when the My Cloud updated to the OS 3 firmware a few months ago. Prior to that one of my drives (a Toshiba 1T) worked, now it doesn’t. To make things even more crazy a 1T drive in a generic docking station I have doesn’t work, yet a 500GB drive in that same docking station does. If you search the forums you’ll find several others with similar problems with certain USB drives not being recognized or usable.