This isn't too unusual. Consumer drives are generally engineered to hide a lot of error recovery activity. E.g., drive tries to read sector. Read fails. Drive retries. Read fails. Repeat N times. Retries can involve different activities (moving the heads around, calibrating, remapping, etc.). All that takes time. If it takes too long, the controller decides the drive has failed and drops it out of the array. Try again a bit later and it may work just fine (and may be fine). "RAID ready" drives expose more of the error/recovery process so the controller can see what's happening.