A bad sector can be easily fixed from Storage Manager – Volume repair. In my case, the repair process started automatically.
The log can be identified as:
DiskStation kernel: [ 5304.730000] res c1/40:50:c8:1a:34/40:00:00:00:00/40 Emask 0x3 (HSM violation)
DiskStation kernel: [ 5304.740000] ata3.00: status: { Busy }
DiskStation kernel: [ 5304.740000] ata3.00: error: { UNC }
DiskStation kernel: [ 5304.750000] ata3.00: failed command: READ FPDMA QUEUED
DiskStation kernel: [ 5304.750000] ata3.00: cmd 60/c8:10:40:3a:42/03:00:00:00:00/40 tag 2 ncq 495616 in
DiskStation kernel: [ 5304.750000] res c1/40:50:c8:1a:34/40:00:00:00:00/40 Emask 0x3 (HSM violation)
DiskStation kernel: [ 5304.760000] ata3.00: status: { Busy }
DiskStation kernel: [ 5304.760000] ata3.00: error: { UNC }
DiskStation kernel: [ 5304.770000] ata3.00: failed command: READ FPDMA QUEUED
DiskStation kernel: [ 5304.770000] ata3.00: cmd 60/98:18:10:3e:42/01:00:00:00:00/40 tag 3 ncq 208896 in
DiskStation kernel: [ 5304.770000] res c1/40:50:c8:1a:34/40:00:00:00:00/40 Emask 0x3 (HSM violation)
DiskStation kernel: [ 5304.780000] ata3.00: status: { Busy }
DiskStation kernel: [ 5304.780000] ata3.00: error: { UNC }
DiskStation kernel: [ 5304.780000] ata3.00: failed command: READ FPDMA QUEUED
DiskStation kernel: [ 5304.790000] ata3.00: cmd 60/e0:20:b0:3f:42/01:00:00:00:00/40 tag 4 ncq 245760 in
DiskStation kernel: [ 5304.790000] res c1/40:50:c8:1a:34/40:00:00:00:00/40 Emask 0x3 (HSM violation)
DiskStation kernel: [ 5304.800000] ata3.00: status: { Busy }
DiskStation kernel: [ 5304.810000] ata3.00: error: { UNC }
DiskStation kernel: [ 5304.810000] ata3.00: failed command: READ FPDMA QUEUED
DiskStation kernel: [ 5304.820000] ata3.00: cmd 60/10:28:60:1d:34/00:00:00:00:00/40 tag 5 ncq 8192 in
DiskStation kernel: [ 5304.820000] res c1/40:50:c8:1a:34/40:00:00:00:00/40 Emask 0x3 (HSM violation)
DiskStation kernel: [ 5304.830000] ata3.00: status: { Busy }