I'd note the serial no of the one thats always having the problem and swap its SATA and power cables to start with. If that makes no difference, after backing up your data, I'd play musical HDDs with it, swapping it to a different port and using a different SATA cable (a reinstall would be necessary). If this same drive still plays up then I'd back up all the data on the RAID volume(s), zero fill the drive and recreate the arrays starting from scratch. It's amazing what a zero fill will do to clear niggly little drive issues.
Since you have 4 identical drives, you can use that to your advantage to see if the problem follows the drive when things are changed around.
Kiwi - that is actually a great idea. Thanks, I will give it a try.
Appreciate the feed back. I didn't eventhink about that.