gparted can't resize paetition

Hi folks,
Situation: I'm replacing the hard drives in my backup server with larger drives (2TB -> 3TB) The drives have one small boot/root partition and one large data partition. Both partitions are RAID1 between the two drives. (At present it is only one drive operating a degraded RAID1 since the other drive failed.)

Steps I have taken:
- bitwise copy of the remaining good 2TB drive to a 3TB drive. (using dd)
- Run gparted to fix the partition table This leaves about 1/3 of the drive unallocated.
- try to use gparted to resize the data partition.

Here's where things go sideways. The only option available when I select the partition I want to resize is to delete it. Option to resize is grayed out. Things I checked/tried
- no partition on the drive is mounted.
- The RAID is not active (/proc/mdstat shows nothing.)
- I tried unsetting the RAID flag and restarting gparted. No change
- reset the RAID flag and tried again.

None of these made any difference.

Is there any hope to resize the RAID partition? I'd really prefer to do that rather than have to stat from scratch with this drive.

Thanks!