These appeared overnight:
Feb 18 08:08:23 knew kernel: (ada0:ata0:0:0:0): READ_DMA48. ACB: 25 00 90 a8 f0 40 4b 00 00 00 f0 00 Feb 18 08:08:23 knew kernel: (ada0:ata0:0:0:0): CAM status: ATA Status Error Feb 18 08:08:23 knew kernel: (ada0:ata0:0:0:0): ATA status: 51 (DRDY SERV ERR), error: 40 (UNC ) Feb 18 08:08:23 knew kernel: (ada0:ata0:0:0:0): RES: 51 40 58 a9 f0 4b 4b 00 00 28 00 Feb 18 08:08:23 knew kernel: (ada0:ata0:0:0:0): Retrying command Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): READ_DMA48. ACB: 25 00 90 a8 f0 40 4b 00 00 00 f0 00 Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): CAM status: ATA Status Error Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): ATA status: 51 (DRDY SERV ERR), error: 40 (UNC ) Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): RES: 51 40 58 a9 f0 4b 4b 00 00 28 00 Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): Retrying command Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): READ_DMA48. ACB: 25 00 90 a8 f0 40 4b 00 00 00 f0 00 Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): CAM status: ATA Status Error Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): ATA status: 51 (DRDY SERV ERR), error: 40 (UNC ) Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): RES: 51 40 58 a9 f0 4b 4b 00 00 28 00 Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): Retrying command Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): READ_DMA48. ACB: 25 00 90 a8 f0 40 4b 00 00 00 f0 00 Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): CAM status: ATA Status Error Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): ATA status: 51 (DRDY SERV ERR), error: 40 (UNC ) Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): RES: 51 40 58 a9 f0 4b 4b 00 00 28 00 Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): Retrying command Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): READ_DMA48. ACB: 25 00 90 a8 f0 40 4b 00 00 00 f0 00 Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): CAM status: ATA Status Error Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): ATA status: 51 (DRDY SERV ERR), error: 40 (UNC ) Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): RES: 51 40 58 a9 f0 4b 4b 00 00 28 00 Feb 18 08:08:38 knew kernel: (ada0:ata0:0:0:0): Error 5, Retries exhausted