A number of years ago, Comcast upgraded the firmware of the DCX3400 to allow the eSATA port to be used with an external hard drive. I briefly tested the "new" feature and found that it indeed worked. I continued in the intervening years without one connected. This morning, I decided to go ahead and connect an unused drive.
Plugged it in with the STB off. With the drive powered on, the STB would not power up. Disconnected the drive and the STB powered up. Brought up the DVR menu, powered the drive up and a banner warning appeared "Remove External Drive". Removed the drive and the STB continued normally.
I give up, has Comcast made another change to the DCX3400 removing the feature?????
Myself and others that I know of, all use 2TB External Drive connected to the DCX3400 with eSATA Cable without any problem.
I would suggest that you power off the DCX3400 and external drive, then connect the eSATA cable between the. Next only owner on the DCX3400. Wait a few minutes to make sure it is working and is able to record to the internal driver. Then power on the external drive. It may take a few minutes for the DCX3400 to see it, and you would get a blue box in the screen saying Drive 1 connected.