You can add USB hard drives to expand storage or to perform a My Cloud backup to the external USB hard drive. See Chapter 12 Connecting a USB Hard Drive in the My Cloud User Manual (http://www.wdc.com/wdproducts/library/?id=439&type=25). If one enabled (sets to On) the “USB Content Availability” option inn the WD My Cloud Dashboard > Settings > General > Cloud Access section, one should see (at least under v4.x firmware versions) the external USB hard drive storage space reflected in the Dashboard Home Capacity section where it is included into the total My Cloud capacity figure.
Others have attached multiple USB drives to their My Cloud, but apparently some have had trouble if they add more than three or four external USB drives.
https://community.wd.com/t/cant-reliably-connect-more-than-3-external-usb-drives-to-my-cloud-at-the-same-time/158195
I’ve attached several external USB hard drives to a USB powered hub and have that hub connected to the My Cloud USB port. I am able to access the USB drives (each drive shows up as a separate Share). Here is how it looks (with “USB Content Availability” enabled) on my 1T v4.x firmware My Cloud. As the screen capture indicates I have a total of 1.4T of free space between the 1T My Cloud and the four external USB hard drives.
