First of all, thanks very much to everyone who replied.
With all of the suggestions, I was able to identify the cause. The disk, the motherboard and OpenBSD were all blameless. The SATA cable had partly worked its way out of the connector. The data connection was just on the edge of going bad. I put a fresh cable in (in case the connector on the old cable was a little loose) but I am not sure if this is a good long term fix. The disk is in a trayless hot-swap mount and I fear that the repeated flexing as I have swapped disks may be the cause. The hotswap cradle merely has a finger connector for the data lead so there is nothing to be gained by using a spring latched connector. I'll just have to hope. Since the system will shortly be off to a lights out data center I guess that I will fret as much as I will hope. Thanks again to everyone for the help. regards jpoc
