MyCloudMirror Gen2 6TB read as 3TB...WHAT?

Setup myCloudMirror Gen2 6TB today, and the www.mycloud.com/setup recognized it as only 3TB. What am I doing wrong?

You are doing nothing wrong. The 6TB spec refers to the total storage capacity of the drive. You have a MyCloudMirror so that 6TB is actually two 3TB hard drives. Those two drives are set up as Raid 1 so each drive is a mirror of the other, if one drive fails you will not lose any data because it will still be on the other drive. If you want or need 6TB of storage, and do not want the redundant security you can change the drive’s Raid setting and access both drives as a single drive with 6TB of storage.