I moved your thread to Chipsets
Intel® Rapid Storage Technology only checks the integrity of the volume and not of the Hard Drive themselves. I recommend asking the hard drive manufacturer for a diagnostic software tool and whether or not this tool allows checking the drive that is part of a RAID array. Otherwise you will need to break the RAID array in order to test each drive for bad sectors. The RAID option ROM also shows failed drives by pressing Ctrl+I during POST.
You can accomplish both goals by replacing all members with higher capacity drives. You will need to rebuild after replacing each one.
You can use Windows* Disk Management to increase the volume size as explained in the Help section of Intel® Rapid Storage Technology; go to search and look for “Increasing Volume Size”.
Adding a fourth drive to the RAID 5 array will not increase the capacity as explained in the table of the RAID migration article. Remember to back up your data before doing any change to the array.