David Greaves wrote:

scsi1 : sata_sil
  Vendor: ATA       Model: Maxtor 6B200M0    Rev: BANC
  Type:   Direct-Access                      ANSI SCSI revision: 05
  Vendor: ATA       Model: Maxtor 6B200M0    Rev: BANC
  Type:   Direct-Access                      ANSI SCSI revision: 05

I wonder if the non-FUA component here is the sata_sil,
rather than the two Maxtor drives.

Also, your drives have different firmware,
but both have trouble with FUA here.

(sdb is slightly newer, and larger, than sda).

Cheers