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I've plugged my WD Easystore 4tb to the usb port and was able to double my storage and move data back and forth.
I would recommend:
Sorry, there was a problem. Please try again later.Based on several YouTube videos like this one https://www.youtube.com/watch?v=OLOavWZfsjQ, it seems like you could do so assuming the My Cloud properly recognizes the drive. But if you have a My Cloud large enough why have another drive? At any rate, it looks like you can do it.
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