When you see the 2TB you are actually seeing both drives, not just one. The two drives are exact copies of each other, so your data is 100% redundant... but it takes up twice the space.
If one of the drives were to die you would still be able to access all of your data, because both drives contain All of your data. (The beauty of RAID 1)
If you were in RAID 0 the you would have 4TB of storage space, but if one of the drives died you would loose access to it all. The trade off obviously is size vs. protection.
The Quick View would alert you that the RAID had failed, then you would just have to get another drive (like Tony described).