Debian – HDD Bad sectors not realloated but no smart errors

I have a pair of WDD drives and recently one got kicked out of the RAID1 array.

SMART is showing that there's IO errors on some sectors, but all the SMART attributes appear to be looking fine:

root@nas:~# smartctl -a /dev/sdb
smartctl 6.5 2016-05-07 r4318 [x86_64-linux-4.4.68.x86_64.1] (local build)
Copyright (C) 2002-16, Bruce Allen, Christian Franke, www.smartmontools.org

=== START OF INFORMATION SECTION ===
Model Family:     Western Digital Red
Device Model:     WDC WD20EFRX-68AX9N0
Serial Number:    WD-WMC30xxxxxxxx
LU WWN Device Id: 5 0014ee 602ce8a27
Firmware Version: 80.00A80
User Capacity:    2,000,398,934,016 bytes [2.00 TB]
Sector Sizes:     512 bytes logical, 4096 bytes physical
Device is:        In smartctl database [for details use: -P show]
ATA Version is:   ACS-2 (minor revision not indicated)
SATA Version is:  SATA 3.0, 6.0 Gb/s (current: 3.0 Gb/s)
Local Time is:    Tue Sep 19 07:50:28 2017 WEST
SMART support is: Available - device has SMART capability.
SMART support is: Enabled

=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED

General SMART Values:
Offline data collection status:  (0x00) Offline data collection activity
                    was never started.
                    Auto Offline Data Collection: Disabled.
Self-test execution status:      ( 121) The previous self-test completed having
                    the read element of the test failed.
Total time to complete Offline
data collection:        (26940) seconds.
Offline data collection
capabilities:            (0x7b) SMART execute Offline immediate.
                    Auto Offline data collection on/off support.
                    Suspend Offline collection upon new
                    command.
                    Offline surface scan supported.
                    Self-test supported.
                    Conveyance Self-test supported.
                    Selective Self-test supported.
SMART capabilities:            (0x0003) Saves SMART data before entering
                    power-saving mode.
                    Supports SMART auto save timer.
Error logging capability:        (0x01) Error logging supported.
                    General Purpose Logging supported.
Short self-test routine
recommended polling time:    (   2) minutes.
Extended self-test routine
recommended polling time:    ( 272) minutes.
Conveyance self-test routine
recommended polling time:    (   5) minutes.
SCT capabilities:          (0x70bd) SCT Status supported.
                    SCT Error Recovery Control supported.
                    SCT Feature Control supported.
                    SCT Data Table supported.

SMART Attributes Data Structure revision number: 16
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  1 Raw_Read_Error_Rate     0x002f   191   191   051    Pre-fail  Always       -       110178
  3 Spin_Up_Time            0x0027   195   170   021    Pre-fail  Always       -       3208
  4 Start_Stop_Count        0x0032   065   065   000    Old_age   Always       -       35326
  5 Reallocated_Sector_Ct   0x0033   200   200   140    Pre-fail  Always       -       0
  7 Seek_Error_Rate         0x002e   200   200   000    Old_age   Always       -       0
  9 Power_On_Hours          0x0032   042   042   000    Old_age   Always       -       43024
 10 Spin_Retry_Count        0x0032   100   100   000    Old_age   Always       -       0
 11 Calibration_Retry_Count 0x0032   100   253   000    Old_age   Always       -       0
 12 Power_Cycle_Count       0x0032   100   100   000    Old_age   Always       -       31
192 Power-Off_Retract_Count 0x0032   200   200   000    Old_age   Always       -       14
193 Load_Cycle_Count        0x0032   189   189   000    Old_age   Always       -       35311
194 Temperature_Celsius     0x0022   120   103   000    Old_age   Always       -       27
196 Reallocated_Event_Count 0x0032   200   200   000    Old_age   Always       -       0
197 Current_Pending_Sector  0x0032   200   200   000    Old_age   Always       -       0
198 Offline_Uncorrectable   0x0030   100   253   000    Old_age   Offline      -       0
199 UDMA_CRC_Error_Count    0x0032   200   200   000    Old_age   Always       -       0
200 Multi_Zone_Error_Rate   0x0008   200   200   000    Old_age   Offline      -       0

SMART Error Log Version: 1
ATA Error Count: 1
    CR = Command Register [HEX]
    FR = Features Register [HEX]
    SC = Sector Count Register [HEX]
    SN = Sector Number Register [HEX]
    CL = Cylinder Low Register [HEX]
    CH = Cylinder High Register [HEX]
    DH = Device/Head Register [HEX]
    DC = Device Command Register [HEX]
    ER = Error register [HEX]
    ST = Status register [HEX]
Powered_Up_Time is measured from power on, and printed as
DDd+hh:mm:SS.sss where DD=days, hh=hours, mm=minutes,
SS=sec, and sss=millisec. It "wraps" after 49.710 days.

Error 1 occurred at disk power-on lifetime: 43000 hours (1791 days + 16 hours)
  When the command that caused the error occurred, the device was active or idle.

  After command completion occurred, registers were:
  ER ST SC SN CL CH DH
  -- -- -- -- -- -- --
  40 51 01 49 00 90 e0  Error: UNC at LBA = 0x00900049 = 9437257

  Commands leading to the command that caused the error were:
  CR FR SC SN CL CH DH DC   Powered_Up_Time  Command/Feature_Name
  -- -- -- -- -- -- -- --  ----------------  --------------------
  40 00 01 49 00 90 e0 08  14d+22:24:56.107  READ VERIFY SECTOR(S)

SMART Self-test log structure revision number 1
Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
# 1  Extended offline    Completed: read failure       90%     42999         9437257
# 2  Short offline       Completed: read failure       40%     42999         9437257
# 3  Extended offline    Completed without error       00%     39200         -
# 4  Extended offline    Completed without error       00%     39033         -
# 5  Extended offline    Completed without error       00%     38864         -
# 6  Extended offline    Completed without error       00%     38708         -
# 7  Extended offline    Completed without error       00%     38540         -
# 8  Extended offline    Completed without error       00%     38396         -
# 9  Short offline       Completed without error       00%         0         -

SMART Selective self-test log data structure revision number 1
 SPAN  MIN_LBA  MAX_LBA  CURRENT_TEST_STATUS
    1        0        0  Not_testing
    2        0        0  Not_testing
    3        0        0  Not_testing
    4        0        0  Not_testing
    5        0        0  Not_testing
Selective self-test flags (0x0):
  After scanning selected spans, do NOT read-scan remainder of disk.
If Selective self-test is pending on power-up, resume after 0 minute delay.

So there's reproducible errors at 9437257, and I can see this with dd:

root@nas:~# export i=9437257
root@nas:~# while [ $i -lt 9437280 ]; do echo $i; dd if=/dev/sdb of=/dev/null bs=512 count=1 skip=$i; let i+=1; done
9437257
dd: error reading '/dev/sdb': Input/output error
0+0 records in
0+0 records out
0 bytes copied, 0.325588 s, 0.0 kB/s
9437258
dd: error reading '/dev/sdb': Input/output error
0+0 records in
0+0 records out
0 bytes copied, 0.164007 s, 0.0 kB/s
9437259
dd: error reading '/dev/sdb': Input/output error
0+0 records in
0+0 records out
0 bytes copied, 0.162149 s, 0.0 kB/s
9437260
dd: error reading '/dev/sdb': Input/output error
0+0 records in
0+0 records out
0 bytes copied, 0.161994 s, 0.0 kB/s
9437261
dd: error reading '/dev/sdb': Input/output error
0+0 records in
0+0 records out
0 bytes copied, 0.161854 s, 0.0 kB/s
9437262
dd: error reading '/dev/sdb': Input/output error
0+0 records in
0+0 records out
0 bytes copied, 0.16294 s, 0.0 kB/s
9437263
dd: error reading '/dev/sdb': Input/output error
0+0 records in
0+0 records out
0 bytes copied, 0.161955 s, 0.0 kB/s
9437264
1+0 records in
1+0 records out
512 bytes copied, 0.0212458 s, 24.1 kB/s
9437265
1+0 records in
1+0 records out
512 bytes copied, 0.000336436 s, 1.5 MB/s
9437266
1+0 records in
1+0 records out
512 bytes copied, 0.000300649 s, 1.7 MB/s
9437267
1+0 records in
1+0 records out
512 bytes copied, 0.000284451 s, 1.8 MB/s
9437268
1+0 records in
1+0 records out
512 bytes copied, 0.00031215 s, 1.6 MB/s
9437269
1+0 records in
1+0 records out
512 bytes copied, 0.000287936 s, 1.8 MB/s
9437270
1+0 records in
1+0 records out
512 bytes copied, 0.000302617 s, 1.7 MB/s
9437271
1+0 records in
1+0 records out
512 bytes copied, 0.000294914 s, 1.7 MB/s
9437272
1+0 records in
1+0 records out
512 bytes copied, 0.000713134 s, 718 kB/s
9437273
1+0 records in
1+0 records out
512 bytes copied, 0.000416336 s, 1.2 MB/s
9437274
1+0 records in
1+0 records out
512 bytes copied, 0.000289526 s, 1.8 MB/s
9437275
1+0 records in
1+0 records out
512 bytes copied, 0.000300769 s, 1.7 MB/s
9437276
1+0 records in
1+0 records out
512 bytes copied, 0.000294524 s, 1.7 MB/s
9437277
1+0 records in
1+0 records out
512 bytes copied, 0.000295592 s, 1.7 MB/s
9437278
1+0 records in
1+0 records out
512 bytes copied, 0.00034751 s, 1.5 MB/s
9437279
1+0 records in
1+0 records out
512 bytes copied, 0.000301789 s, 1.7 MB/s
root@nas:~#

I've tried writing to these sectors to get them reassigned, but I just get more errors:

root@nas:~# dd if=/dev/zero of=/dev/sdb bs=512 count=7 seek=9437257
dd: error writing '/dev/sdb': Input/output error
1+0 records in
0+0 records out
0 bytes copied, 0.168565 s, 0.0 kB/s
root@nas:~#

I've also tried sg_verify and sg_reassign:

root@nas:~# sg_verify /dev/sdb --lba=9437257
verify (10):
Descriptor format, current; Sense key: Medium Error
Additional sense: Unrecovered read error - auto reallocate failed
  Descriptor type: Information: 0x0000000000900049
VERIFY(10) medium or hardware error, reported lba=0x900049
root@nas:~# sg_reassign --address=9437257 /dev/sdb
REASSIGN BLOCKS: Illegal request, invalid opcode

There's also plenty of noise in journalctl:

Sep 19 07:58:26 nas kernel: ata2.00: configured for UDMA/133
Sep 19 07:58:26 nas kernel: sd 1:0:0:0: [sdb] tag#12 FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
Sep 19 07:58:26 nas kernel: sd 1:0:0:0: [sdb] tag#12 Sense Key : Medium Error [current] [descriptor]
Sep 19 07:58:26 nas kernel: sd 1:0:0:0: [sdb] tag#12 Add. Sense: Unrecovered read error - auto reallocate failed
Sep 19 07:58:26 nas kernel: sd 1:0:0:0: [sdb] tag#12 CDB: Read(10) 28 00 00 90 00 49 00 00 01 00
Sep 19 07:58:26 nas kernel: blk_update_request: I/O error, dev sdb, sector 9437257
Sep 19 07:58:26 nas kernel: Buffer I/O error on dev sdb3, logical block 9, async page read
Sep 19 07:58:26 nas kernel: ata2: EH complete
Sep 19 07:58:26 nas kernel: ata2.00: exception Emask 0x0 SAct 0x1f800000 SErr 0x0 action 0x0
Sep 19 07:58:26 nas kernel: ata2.00: irq_stat 0x40000008
Sep 19 07:58:26 nas kernel: ata2.00: failed command: READ FPDMA QUEUED
Sep 19 07:58:26 nas kernel: ata2.00: cmd 60/01:e0:4a:00:90/00:00:00:00:00/40 tag 28 ncq 512 in
                                     res 41/40:00:4a:00:90/00:00:00:00:00/40 Emask 0x409 (media error) <F>
Sep 19 07:58:26 nas kernel: ata2.00: status: { DRDY ERR }
Sep 19 07:58:26 nas kernel: ata2.00: error: { UNC }

So my question is: Is there a reason that auto reallocation is failing, and is it recoverable. As I said, the data is (was) protected by RAID, so I'm not worried about data recovery.