I had a bit of a look at dmesg, I didn't google all the errors however to me it looks like you possibly have a bad cable or SATA port. You could try swapping the SATA cables and ports (although, don't do it if the RAID isn't rebuilt!) and see if the error follows the drive, cable or SATA port.
There is also some suggestion it might be the mode the hardware is running in or some kind of power management problem.
I looked a bit more into dmesg and the SATA cables as you suggested. I had originally just checked the connectors, but didn't think too much of the kink on the cable itself. Turns out straightening the cable did the trick. I've ordered some new cables and will replace the dodgy one, but the raid is now in active sync for the last 4 days!
Cheers
John