PR4100 questions

I’m looking at potentially buying the PR4100, but after reading the manual I can’t answer certain questions

  1. What is the process when a drive fails
  2. Must the replacement drive be identical?
  3. Can you migrate to larger drives in the future.
  4. Is there no control over fan speed?
  5. What’s the best low cost option for whole NAS backup?
  6. Is there any benefit/advantage to using WD or HGST drives?
  7. When the NAS is not being accessed, will the drives sleep(stop spinning)? Is there any config for this?