Help understanding the root cause of a member dropping out of a RAID 1 set.

I am running RAID1 partitions on some systems and a few times I have seen a raid set become degraded as a member has failed out of the md device.  Looking at the /var/log/message file I have seen output similar to below:

Can anyone help me decode what actually happened here.  

Thanks Simon.

2009-08-11T06:21:04-07:00 Metro-1 kernel: [556568.670377]          res 40/00:00:00:00:00/00:00:00:00:00/00 Emask 0x4 (timeout)
2009-08-11T06:21:04-07:00 Metro-1 kernel: [556568.670477] ata1: hard resetting link
2009-08-11T06:21:08-07:00 Metro-1 kernel: [556573.122562] ata1: SATA link up 3.0 Gbps (SStatus 123 SControl 300)
2009-08-11T06:21:08-07:00 Metro-1 kernel: [556573.259057] ata1.00: configured for UDMA/133
2009-08-11T06:21:08-07:00 Metro-1 kernel: [556573.348168] ata1.01: configured for UDMA/133
2009-08-11T06:21:08-07:00 Metro-1 kernel: [556573.348168] md: super_written gets error=-5, uptodate=0
2009-08-11T06:21:08-07:00 Metro-1 kernel: [556573.348168] raid1: Operation continuing on 1 devices.
2009-08-11T06:21:08-07:00 Metro-1 kernel: [556573.348168] ata1: EH complete