How to backup on the 2nd drive?

I have a WD My Cloud EX2 with 2 drives of 2TB each.
How do I backup drive 1 to drive 2?
if this is automatic, I can only see 1 drive when networked in?

So, the backup is built in.
You cannot access drive 2 via a folder, its a ghost backup.
If drive 1 goes down, your files will still be available as the connection will be viewing drive 2.

Depends on which RAID mode you select. If you select JBOD (‘just a bunch of disks’), you won’t get any redundancy, but all the disks will be available for user storage.

BTW, you may be better posting in the EX2 forum:

Thanks, Raid 1 is set to default, which will use the other drive as a backup.