Hard drives can fail from vibrations, power fluctuations, and errors during automatic diagnostics. Other problems like inconsistent data in cache can make drives fail in a RAID array.
/dev/sda2 is just a partition. It could be the partition table is going.
You may want to get an add-on for your controller that uses a Silicon Image 4723 chip that does hardware RAID-1. The following has the chip that should work with your controller and with Linux.
If that is not feasible, I suggest activate logging for the controller and mdadm.