Yes, I believe you can, since My Cloud comes with a WD green drive and is listed under compatible drives here -> http://www.wdc.com/en/products/products.aspx?id=1180#tab9
To be sure about what drive model is inside your My Cloud, you can run this command on your old My Cloud via an SSH session:
hdparm -i /dev/sda (I’m guessing the drive is under the device /dev/sda in all probability…if that doesn’t show up anything first run fdisk -l to see under what device is the hard drive listed)
Having said that, I strongly recommend you don’t re-use the green drive in your EX2 as its ideal usage is for desktop use, but rather buy red drives whose firmware are optimized for NAS usage. Using a compatible drive like a green one will work, but may reduce the life of the drive…and it might fail sooner than expected, depending on your NAS usage. Why go that route anyway? BTW, if you did buy an EX2 with disks, they do come with WD red drives.