I have My Cloud NAS - WDBCTL0040HWT-NESN I have a 4 port USB powered hub plugged in with 3 8 terabyte drives plugged in which work fine. I tried plugging in a 4th drive it shows up on web page with zero bytes 
if there a limit ?
You need to look at the User Manual for your generation of My Cloud. See Chapter 12.
Found the problem I had to eject the drive unplug it and replug it back in now it show terabytes
It helps to use a powered USB hub or powered USB hard drives when using multiple USB drives with the My Cloud. Otherwise one is relying on the My Cloud USB port to power both the USB Hub and all drives connected to it.
While the single bay My Cloud documentation indicates one can use up to seven USB drives some of us have had trouble using more than four unpowered USB drives and an unpowered USB hub.
