Hello
This is with a new drive, new motherboard, SATA II.  The machine seems 
to be fine until it gets loaded my a xen guest domain, at which point 
disk access stalled - presumably because this is happening:
Dec 13 10:36:03 [kernel] ata3: handling error/timeout
Dec 13 10:36:03 [kernel] ata3: port reset, p_is 1 is 4 pis 1 cmd c017 tf 
50 ss 123 se 0
Dec 13 10:36:03 [kernel] ata3: status=0x50 { DriveReady SeekComplete }
Dec 13 10:36:03 [kernel] sda: Current: sense key=0x0
Dec 13 10:36:03 [kernel]     ASC=0x0 ASCQ=0x0
Dec 13 10:36:33 [kernel] ata3: handling error/timeout
Dec 13 10:36:33 [kernel] ata3: port reset, p_is 1 is 4 pis 1 cmd c017 tf 
50 ss 123 se 0
Dec 13 10:36:33 [kernel] ata3: status=0x50 { DriveReady SeekComplete }
Dec 13 10:36:33 [kernel] sda: Current: sense key=0x0
Dec 13 10:36:33 [kernel]     ASC=0x0 ASCQ=0x0
Does anyone have any good ideas?  (For example, it's a Intel 6300 DC - 
is there a way to turn off one of the processors to test?)
Cheers
James