When using RAID 1 it is required to read data stored on the second member disk as well. If you do not check the second disk by reading data off it on a regular basis, then it may lead to that a bad sector appears on the second member disk of the RAID1. In this case your RAID 1 doesn’t provide redundancy any longer and on top of that you do not know it. It should be noted that the same is true for hot spare drives as well.
In most cases this problem occurs if your RAID 1 is not heavily used most of the time. To reduce the risk one should read data from both drives and compare it. Heavily loaded arrays don’t experience such issue.
It is known that in RAID 1 used by Windows OS sectors are usually requested from the first disk. This fact makes Windows RAID 1 not reliable.