Drive keep dropping out when backing up

I have two 3 TB My Books hooked up.

I have tons of data stored on one and am trying to back up to the ohter.

After about 10 minutes of backing up, the back up drive drops and then comes right back and of course the backup gives errors. When I first got these two drives the backup work fine the afteer a coule weeks this starting to happpen.

What error would that be? Also are both drives directly connected to a wall outlet? If not try that first.