When using RAID1 it is required to access data on the second disk as well. If you do not, then one or even several bad sectors may develop on the second member disk of the RAID 1.
In this case the RAID 1 does not have fault-tolerance any longer and on top of that you do not know about it. Usually this situation occurs if the RAID 1 is idle most of the time. To make sure there is still fault tolerance you need to read data from two hard disks and compare it. Heavily loaded arrays don't have this problem.
It can be shown that in RAID1 used by Windows data is typically read from the first disk. This quirk renders Windows RAID1 unreliable.
It is interesting that hot spare drives are also subject to the same problem; therefore they are needed to be tested as well.
