What do "doesn't support DPO or FUA" and other disk cache messages mean ?

Issue

• When booting a system, the messages below about device caching are always output.
• When adding a disk to the system, the messages below are output during disk discovery.
• When plugging in a USB disk to the system, the messages below are output during discovery.
• Please explain what the following boot time messages mean:

sd 5:0:0:0: [sda] 209715200 512-byte logical blocks: (107 GB/100 GiB)
sd 5:0:0:0: [sda] Write Protect is off
sd 5:0:0:0: [sda] Mode Sense: 9f 00 00 00
sd 5:0:0:0: [sda] Write cache: disabled, read cache: disabled, doesn't support DPO or FUA

sd 2:0:0:0: [sdb] 488281250 512-byte logical blocks: (250 GB/232 GiB)
sd 2:0:0:0: [sdb] Write Protect is off
sd 2:0:0:0: [sdb] Mode Sense: 00 3a 00 00
sd 2:0:0:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA

scsi9 : SCSI emulation for USB Mass Storage devices
scsi 9:0:0:0: Direct-Access     WDC WD32 00AAJS-00L7A0         PQ: 0 ANSI: 2 CCS
sd 9:0:0:0: Attached scsi generic sg3 type 0
sd 9:0:0:0: [sdc] 625142448 512-byte logical blocks: (320 GB/298 GiB)
sd 9:0:0:0: [sdc] Write Protect is off
sd 9:0:0:0: [sdc] Assuming drive cache: write through

scsi 1:0:0:0: Direct-Access     DGC      LUNZ             0532 PQ: 0 ANSI: 4
sd 1:0:0:0: [sdb] READ CAPACITY failed
sd 1:0:0:0: [sdb]  Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
sd 1:0:0:0: [sdb]  Sense Key : Illegal Request [current]
sd 1:0:0:0: [sdb]  Add. Sense: Logical unit not supported
sd 1:0:0:0: Attached scsi generic sg2 type 0
sd 1:0:0:0: [sdb] Test WP failed, assume Write Enabled
sd 1:0:0:0: [sdb] Asking for cache data failed
sd 1:0:0:0: [sdb] Assuming drive cache: write through

{virtual disk}
sd 0:0:0:3: [sdd] 73400320 512-byte logical blocks: (37.5 GB/35.0 Gib)
sd 0:0:0:3: [sdd] Write Protect is off
sd 0:0:0:3: [sdd] Mode Sense: 61 00 00 00
sd 0:0:0:3: [sdd] Cache data unavailable
sd 0:0:0:3: [sdd] Assuming drive cache: write through

{on console during boot}
Asking for cache data failed
Assuming drive cache : write through

• Receiving message "doesn't support DPO or FUA", what does this mean?
• Why is disk write cache shown as disabled in "Write cache: enabled, read cache: enabled"?
• Receiving message "Assuming drive cache : write through", what does that mean?
• Do any of these messages mean there are problems with storage?