ReadyNAS Duo V2 issue - attempting to increase storage with 3TB drive

I have had my ReadyNas Duo v2 fro 4 years. I am running RAIDar 4.3.8. Firmware Revision Status shows 4.1.14.

On the RAIDar screen it shows It has 2 1TB Seagate Pipeline drives. They are fully redundant. I want to expand to 3TB of storage and have purchased 2 3TH WD Red drives (WD30EFRX). I pullled the drive from slot 1 and replaced it with one of the 3TB Drives. It went through the Re-sync process and, when it was finished I would have expected to have 2 drives, redundant at 931GB capacity. Instead, it lists drive 1 as Dead. So I pulled that drive and re-inserted the original 1TB Seagate drive. That re-initialized and showed the system to be fully redundant.

Next, I tried using the other 3TB drive (to make sure I didn't have a bad drive from WD) and tried the same operation using slot 2 this time. Same issue. System says it is not redundant and that the drive in Slot 2 is Dead. Interestingly, it thinks it is still a Seagate drive. RAIDar says "Channel 2: Seagate ST310005528AS 931GB, 0C/32F[Dead]

The log says, "The disk attached to channel 2 could not be used. The most common reasons are RAID resync in progress, faulty drives, and disks that are too small to be added to the array." 

Any suggestions? What am I doing wrong? What should I try next?