I tried running preclear again on the disk and once again it failed. This is what I found in the disk log information, does it help?
Jan 7 08:58:36 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 7 08:58:36 unNAS kernel: sd 10:0:0:0: [sdl] 27344764928 512-byte logical blocks: (14.0 TB/12.7 TiB)
Jan 7 08:58:36 unNAS kernel: sd 10:0:0:0: [sdl] Write Protect is off
Jan 7 08:58:36 unNAS kernel: sd 10:0:0:0: [sdl] Mode Sense: 03 00 00 00
Jan 7 08:58:36 unNAS kernel: sd 10:0:0:0: [sdl] No Caching mode page found
Jan 7 08:58:36 unNAS kernel: sd 10:0:0:0: [sdl] Assuming drive cache: write through
Jan 7 08:58:36 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 7 08:58:36 unNAS kernel: sd 10:0:0:0: [sdl] Attached SCSI disk
Jan 7 08:58:39 unNAS emhttpd: WDC_WD14_0EFGX-68B0GN0_PROLIFICMP000000001-0:0 (sdl) 512 27344764928
Jan 7 08:59:55 unNAS emhttpd: spinning up /dev/sdl
Jan 7 08:59:55 unNAS emhttpd: read SMART /dev/sdl
Jan 7 12:59:56 unNAS emhttpd: spinning down /dev/sdl
Jan 7 15:52:06 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 7 15:52:06 unNAS kernel: sd 10:0:0:0: [sdl] 27344764928 512-byte logical blocks: (14.0 TB/12.7 TiB)
Jan 7 15:52:06 unNAS kernel: sd 10:0:0:0: [sdl] Write Protect is off
Jan 7 15:52:06 unNAS kernel: sd 10:0:0:0: [sdl] Mode Sense: 03 00 00 00
Jan 7 15:52:06 unNAS kernel: sd 10:0:0:0: [sdl] No Caching mode page found
Jan 7 15:52:06 unNAS kernel: sd 10:0:0:0: [sdl] Assuming drive cache: write through
Jan 7 15:52:06 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 7 15:52:06 unNAS kernel: sd 10:0:0:0: [sdl] Attached SCSI disk
Jan 7 15:55:12 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 7 15:55:57 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 7 15:55:57 unNAS kernel: sd 10:0:0:0: [sdl] 27344764928 512-byte logical blocks: (14.0 TB/12.7 TiB)
Jan 7 15:55:57 unNAS kernel: sd 10:0:0:0: [sdl] Write Protect is off
Jan 7 15:55:57 unNAS kernel: sd 10:0:0:0: [sdl] Mode Sense: 03 00 00 00
Jan 7 15:55:57 unNAS kernel: sd 10:0:0:0: [sdl] No Caching mode page found
Jan 7 15:55:57 unNAS kernel: sd 10:0:0:0: [sdl] Assuming drive cache: write through
Jan 7 15:55:57 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 7 15:55:57 unNAS kernel: sd 10:0:0:0: [sdl] Attached SCSI disk
Jan 8 08:57:02 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 8 08:57:02 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 8 11:08:05 unNAS kernel: sd 10:0:0:0: [sdl] tag#0 UNKNOWN(0x2003) Result: hostbyte=0x00 driverbyte=DRIVER_OK cmd_age=0s
Jan 8 11:08:05 unNAS kernel: sd 10:0:0:0: [sdl] tag#0 Sense Key : 0x5 [current]
Jan 8 11:08:05 unNAS kernel: sd 10:0:0:0: [sdl] tag#0 ASC=0x20 ASCQ=0x0
Jan 8 11:08:05 unNAS kernel: sd 10:0:0:0: [sdl] tag#0 CDB: opcode=0x8a 8a 00 00 00 00 00 c1 98 68 00 00 00 08 00 00 00
Jan 8 11:08:05 unNAS kernel: critical target error, dev sdl, sector 3247990784 op 0x1:(WRITE) flags 0x104000 phys_seg 256 prio class 0
Jan 8 11:08:05 unNAS kernel: Buffer I/O error on dev sdl, logical block 405998848, lost async page write
Jan 8 11:08:05 unNAS kernel: Buffer I/O error on dev sdl, logical block 405998849, lost async page write
Jan 8 11:08:05 unNAS kernel: Buffer I/O error on dev sdl, logical block 405998850, lost async page write
Jan 8 11:08:05 unNAS kernel: Buffer I/O error on dev sdl, logical block 405998851, lost async page write
Jan 8 11:08:05 unNAS kernel: Buffer I/O error on dev sdl, logical block 405998852, lost async page write
Jan 8 11:08:05 unNAS kernel: Buffer I/O error on dev sdl, logical block 405998853, lost async page write
Jan 8 11:08:05 unNAS kernel: Buffer I/O error on dev sdl, logical block 405998854, lost async page write
Jan 8 11:08:05 unNAS kernel: Buffer I/O error on dev sdl, logical block 405998855, lost async page write
Jan 8 11:08:05 unNAS kernel: Buffer I/O error on dev sdl, logical block 405998856, lost async page write
Jan 8 11:08:05 unNAS kernel: Buffer I/O error on dev sdl, logical block 405998857, lost async page write
Jan 9 08:33:06 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 9 08:34:05 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 9 08:34:06 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 9 08:34:06 unNAS kernel: sdl: sdl1
Jan 9 08:34:06 unNAS unassigned.devices: Partition 'sdl1' does not have a file system and cannot be mounted.
Jan 9 08:34:07 unNAS emhttpd: read SMART /dev/sdl
Jan 9 08:45:58 unNAS unassigned.devices: Removing all partitions from disk '/dev/sdl'.
Jan 9 08:46:00 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 9 08:46:02 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 9 08:51:16 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 9 08:51:16 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 9 12:34:08 unNAS emhttpd: spinning down /dev/sdl
Jan 10 08:27:16 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 10 08:28:16 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 10 08:28:17 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 10 08:28:17 unNAS kernel: sdl: sdl1
Jan 10 08:28:17 unNAS unassigned.devices: Partition 'sdl1' does not have a file system and cannot be mounted.
Jan 10 10:16:49 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 10 10:16:49 unNAS kernel: sd 10:0:0:0: [sdl] 27344764928 512-byte logical blocks: (14.0 TB/12.7 TiB)
Jan 10 10:16:49 unNAS kernel: sd 10:0:0:0: [sdl] Write Protect is off
Jan 10 10:16:49 unNAS kernel: sd 10:0:0:0: [sdl] Mode Sense: 03 00 00 00
Jan 10 10:16:49 unNAS kernel: sd 10:0:0:0: [sdl] No Caching mode page found
Jan 10 10:16:49 unNAS kernel: sd 10:0:0:0: [sdl] Assuming drive cache: write through
Jan 10 10:16:49 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 10 10:16:49 unNAS kernel: sdl: sdl1
Jan 10 10:16:49 unNAS kernel: sd 10:0:0:0: [sdl] Attached SCSI disk
Jan 10 10:16:51 unNAS unassigned.devices: Partition 'sdl1' does not have a file system and cannot be mounted.
Jan 10 10:18:12 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 10 10:18:12 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 11 09:54:13 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 11 09:54:58 unNAS emhttpd: read SMART /dev/sdl
Jan 11 09:55:12 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 11 09:55:13 unNAS kernel: sd 10:0:0:0: [sdl] Very big device. Trying to use READ CAPACITY(16).
Jan 11 09:55:13 unNAS kernel: sdl: sdl1
Jan 11 09:55:13 unNAS unassigned.devices: Partition 'sdl1' does not have a file system and cannot be mounted.
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