Aug 9 07:41:29 monet kernel: ata1.00: limiting speed to UDMA/66:PIO4
Aug 9 07:41:29 monet kernel: ata1.00: exception Emask 0x0 SAct 0x0 SErr
0x0 action 0x2
Aug 9 07:41:29 monet kernel: ata1.00: (BMDMA stat 0x64)
Aug 9 07:41:29 monet kernel: ata1.00: cmd
c8/00:08:fb:7c:a1/00:00:00:00:00/e0 tag 0 cdb 0x12 data 4096 in
Aug 9 07:41:29 monet kernel: res
51/84:00:00:00:00/00:00:00:00:00/e0 Emask 0x10 (ATA bus error)
Aug 9 07:41:29 monet kernel: ata1: soft resetting port
Aug 9 07:41:29 monet kernel: ata1.00: configured for UDMA/66
Aug 9 07:41:29 monet kernel: ata1.01: configured for UDMA/33
Aug 9 07:41:29 monet kernel: ata1: EH complete
Drops to UDMA33 for 40 wire cable.
Looks to me like your cabling is out of spec or you have electrical noise
problems.