Disk dropped offline and then reconnected with a different ID:
Aug 17 03:41:35 NAS kernel: sd 5:0:4:0: attempting task abort! scmd(00000000389ba74e)
Aug 17 03:41:35 NAS kernel: sd 5:0:4:0: [sdf] tag#118 CDB: opcode=0x35 35 00 00 00 00 00 00 00 00 00
Aug 17 03:41:35 NAS kernel: scsi target5:0:4: handle(0x000e), sas_address(0x50030480013831f5), phy(21)
Aug 17 03:41:35 NAS kernel: scsi target5:0:4: enclosure logical id(0x50030480013831ff), slot(9)
Aug 17 03:41:35 NAS kernel: sd 5:0:4:0: device_block, handle(0x000e)
Aug 17 03:41:37 NAS kernel: sd 5:0:4:0: device_unblock and setting to running, handle(0x000e)
Aug 17 03:41:37 NAS kernel: sd 5:0:4:0: [sdf] Synchronizing SCSI cache
Aug 17 03:41:37 NAS kernel: sd 5:0:4:0: [sdf] Synchronize Cache(10) failed: Result: hostbyte=0x01 driverbyte=0x00
Aug 17 03:41:37 NAS rc.diskinfo[8980]: SIGHUP received, forcing refresh of disks info.
Aug 17 03:41:38 NAS kernel: scsi 5:0:4:0: task abort: SUCCESS scmd(00000000389ba74e)
Aug 17 03:41:38 NAS kernel: scsi 5:0:4:0: rejecting I/O to dead device
Could be a connection issue, but SMART is also showing some issues, you should run an extended SMART test, also see here for better pool monitoring.