Most of the time, you may got soft media errors [bad block], I don't know the exact number of the threshold, FLARE will kick the hotspare in with the event number 803 [Recommend disk replacement] and will eventually proactively replace the issue disk automatically. That's one of the feature of FLARE.
Recently, I noticed one disk reported several soft media errors in the event log, is this a problem? Do we need to replace this disk immediately?
soft media errors observed