Ultra 6 Replacement Drive Not Syncing

I've been running a ReadyNAS Ultra 6 Plus (latest official firmware) for some six years (and an NV+ for years before that), so when I got messages that one of the six 2-TByte drives (Seagate ST2000DL003-9VT166) was rapidly accumulating SMART+ errors and shortly thereafter died, that wasn't a big shock.  I had a 4-TByte drive (Seagate ST4000VN000-1H4168) on  standby and just replaced the faulty drive. (*)

(*) Yes, I realize that replacing a single 2TB drive with a 4TB will not yield any storage benefits.  And right now I do not need more space.  However, anticipating that I will need more space in the long run, I switched to 4TB drives as standbys.

The problem is that the ReadyNAS does not appear to make any attempt to sync the new drive.  The front panel (which in the past, I think, indicated syncing progress) says nothing except array unprotected.  Frontview still lists the old hard drive as being plugged in and dead.

I tested the replacement drive in a workstation and it appears to work fine.  Moreover, once I sshd into the NAS, the logs indicated that the NAS had recognized the new drive just fine.  For example, syslog contains the following lines:

Jan 27 16:41:44 NASNAME RAIDiator: Disk removal detected. [Disk  2]
Jan 27 16:41:44 NASNAME RAIDiator: A disk was removed from the ReadyNAS.  One or more RAID volumes are currently unprotected, and an additional disk failure or removal may result in data loss.  Please add a replacement disk as soon as possible.
Jan 27 16:41:51 NASNAME RAIDiator: Disk removal detected. [Disk  2] (NASNAME) : A disk was removed from the ReadyNAS.  One or more RAID volumes are currently unprotected, and an additional disk failure or removal may result in data loss.  Please add a replacement disk as soon as possible.
Jan 27 16:54:29 NASNAME kernel: ata2: exception Emask 0x10 SAct 0x0 SErr 0x4040000 action 0xe frozen
Jan 27 16:54:29 NASNAME kernel: ata2: irq_stat 0x00000040, connection status changed
Jan 27 16:54:29 NASNAME kernel: ata2: SError: { CommWake DevExch }
Jan 27 16:54:29 NASNAME kernel: ata2: hard resetting link
Jan 27 16:54:35 NASNAME kernel: ata2: link is slow to respond, please be patient (ready=0)
Jan 27 16:54:39 NASNAME kernel: ata2: COMRESET failed (errno=-16)
Jan 27 16:54:39 NASNAME kernel: ata2: hard resetting link
Jan 27 16:54:43 NASNAME kernel: ata2: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
Jan 27 16:54:43 NASNAME kernel: ata2.00: ATA-9: ST4000VN000-1H4168, SC46, max UDMA/133
Jan 27 16:54:43 NASNAME kernel: ata2.00: 7814037168 sectors, multi 0: LBA48 NCQ (depth 31/32), AA
Jan 27 16:54:43 NASNAME kernel: ata2.00: configured for UDMA/133
Jan 27 16:54:43 NASNAME kernel: ata2: EH complete
Jan 27 16:54:43 NASNAME kernel: scsi 1:0:0:0: Direct-Access     ATA      ST4000VN000-1H41 SC46 PQ: 0 ANSI: 5
Jan 27 16:54:43 NASNAME kernel: sd 1:0:0:0: [sdb] 7814037168 512-byte logical blocks: (4.00 TB/3.63 TiB)
Jan 27 16:54:43 NASNAME kernel: sd 1:0:0:0: [sdb] 4096-byte physical blocks
Jan 27 16:54:43 NASNAME kernel: sd 1:0:0:0: [sdb] Write Protect is off
Jan 27 16:54:43 NASNAME kernel: sd 1:0:0:0: [sdb] Mode Sense: 00 3a 00 00
Jan 27 16:54:43 NASNAME kernel: sd 1:0:0:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
Jan 27 16:54:43 NASNAME kernel: sd 1:0:0:0: Attached scsi generic sg1 type 0
Jan 27 16:54:43 NASNAME kernel:  sdb: sdb1
Jan 27 16:54:43 NASNAME kernel: sd 1:0:0:0: [sdb] Attached SCSI disk
Jan 27 16:54:49 NASNAME RAIDiator: New disk detected.  If multiple disks have been added, they will be processed one at a time.  Please do not remove any added disk(s) during this time. [Disk  2]
Jan 27 16:54:51 NASNAME kernel:  sdb: unknown partition table
Jan 27 16:54:53 NASNAME kernel:  sdb: sdb1 sdb2
Jan 27 16:54:59 NASNAME RAIDiator: New disk detected. [Disk  2] (NASNAME) : A new disk was added to the ReadyNAS.  If multiple disks have been added, they will be processed one at a time.  Please do not remove any added disk(s) during this time.
Jan 27 16:56:02 NASNAME kernel:  sdb: unknown partition table
Jan 27 16:56:02 NASNAME last message repeated 2 times
Jan 27 16:56:04 NASNAME kernel:  sdb:

However, after that nothing; no indication that the sync is actually being performed.  The "unknown partition table" line led me to try this:

# sgdisk -p /dev/sda
Disk /dev/sda: 3907029168 sectors, 1.8 TiB
Logical sector size: 512 bytes
Disk identifier (GUID): 1B152AA5-FB31-4410-9B3D-5EA80C3C28EA
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 3907029134
Partitions will be aligned on 8-sector boundaries
Total free space is 5108 sectors (2.5 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1              64         8388671   4.0 GiB     FD00  Linux RAID
   2         8388672         9437247   512.0 MiB   FD00  Linux RAID
   5         9437256      3907024064   1.8 TiB     FD00  Linux RAID

# sgdisk -p /dev/sdb
Disk /dev/sdb: 7814037168 sectors, 3.6 TiB
Logical sector size: 512 bytes
Disk identifier (GUID): E4FEAF3C-6DB0-4E7C-8AA0-F94C353BDB81
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 7814037134
Partitions will be aligned on 2048-sector boundaries
Total free space is 7814037101 sectors (3.6 TiB)

Number  Start (sector)    End (sector)  Size       Code  Name

However, I hesitate to go any further, lest I brick the system.  Any advice?