It looks like your disk dropped off-line and then came back with a different device designation:
Apr 9 13:50:33 DJW-UNRAID kernel: I/O error, dev sdb, sector 7814036768 op 0x0:(READ) flags 0x80700 phys_seg 1 prio class 0
Apr 9 13:50:33 DJW-UNRAID kernel: sd 5:0:12:0: [sdb] tag#686 UNKNOWN(0x2003) Result: hostbyte=0x00 driverbyte=DRIVER_OK cmd_age=0s
Apr 9 13:50:33 DJW-UNRAID kernel: sd 5:0:12:0: [sdb] tag#686 Sense Key : 0x2 [current]
Apr 9 13:50:33 DJW-UNRAID kernel: sd 5:0:12:0: [sdb] tag#686 ASC=0x4 ASCQ=0x0
Apr 9 13:50:33 DJW-UNRAID kernel: sd 5:0:12:0: [sdb] tag#686 CDB: opcode=0x88 88 00 00 00 00 01 d1 c0 bd 20 00 00 00 08 00 00
Apr 9 13:50:33 DJW-UNRAID kernel: I/O error, dev sdb, sector 7814036768 op 0x0:(READ) flags 0x0 phys_seg 1 prio class 0
Apr 9 13:50:33 DJW-UNRAID kernel: Buffer I/O error on dev sdb, logical block 976754596, async page read
Apr 9 13:50:38 DJW-UNRAID kernel: XFS: metadata IO error: 5 callbacks suppressed
Apr 9 13:50:38 DJW-UNRAID kernel: XFS (sdh1): metadata I/O error in "xfs_da_read_buf+0x9a/0xff [xfs]" at daddr 0xebc5fc18 len 8 error 5
Apr 9 13:50:38 DJW-UNRAID kernel: XFS (sdh1): metadata I/O error in "xfs_da_read_buf+0x9a/0xff [xfs]" at daddr 0x15e8c56e8 len 8 error 5
Apr 9 13:50:38 DJW-UNRAID kernel: XFS (sdh1): metadata I/O error in "xfs_da_read_buf+0x9a/0xff [xfs]" at daddr 0x15e8c56f0 len 8 error 5
Apr 9 13:50:38 DJW-UNRAID kernel: XFS (sdh1): metadata I/O error in "xfs_da_read_buf+0x9a/0xff [xfs]" at daddr 0x15e8c56f8 len 8 error 5
Apr 9 13:50:38 DJW-UNRAID kernel: XFS (sdh1): metadata I/O error in "xfs_da_read_buf+0x9a/0xff [xfs]" at daddr 0x15e8c5700 len 8 error 5
Apr 9 13:50:38 DJW-UNRAID kernel: XFS (sdh1): metadata I/O error in "xfs_da_read_buf+0x9a/0xff [xfs]" at daddr 0x15e8c5708 len 8 error 5
Apr 9 13:50:38 DJW-UNRAID kernel: XFS (sdh1): metadata I/O error in "xfs_da_read_buf+0x9a/0xff [xfs]" at daddr 0x15e8c5710 len 8 error 5
Apr 9 13:50:38 DJW-UNRAID kernel: XFS (sdh1): metadata I/O error in "xfs_da_read_buf+0x9a/0xff [xfs]" at daddr 0x15e8c5718 len 8 error 5
Apr 9 13:50:38 DJW-UNRAID kernel: XFS (sdh1): metadata I/O error in "xfs_da_read_buf+0x9a/0xff [xfs]" at daddr 0x15e8c5720 len 8 error 5
Apr 9 13:50:38 DJW-UNRAID kernel: XFS (sdh1): metadata I/O error in "xfs_da_read_buf+0x9a/0xff [xfs]" at daddr 0x15e8c5728 len 8 error 5
The disk is mounted as sdh1 and not sdb1:
/dev/sdh1 3.7T 2.9T 785G 79% /mnt/disks/WD-WCC4E0978614
/dev/sdg1 3.7T 2.7T 972G 74% /mnt/disks/WD-WMC4E0027693
A reboot is needed to clear this up.