"Reallocated sector count has increased" email notification

Knock on wood, but my replacement drive arrived today and I am 50% into the estimated 4 hr 45 min resync process. So far, from searching for and asking questions here, to calling in to tech support, to getting the replacement ordered through warranty, to receiving the disk via UPS 2nd day to simply slipping the new (re-cerrtified) drive into the caddy then into the RNPP unit, to the automatic disk test and now the resync. Now, I am glad I paid the nice price for this machine.

Just a quick check of the three still-working drives. All average about 10 hours on time/day for the past 2 years 5 months. Is this one drive failure (at least starting to degrade quickly) out of four (identical model and purchased together) reasonable for this three-year warranty, non-enterprise drive?