When I was away in Hong Kong we had a particularly nasty black out. This caused one of the drives in my Qnap to develop bad blocks. Not a problem really because I bought the Qnap with 4 drives in total to replace the drives in the machine when they go belly up. The Qnap only uses 2 at a time. I thought the procedure would be difficult. It is not. It is brilliantly simple.
Without turning off the machine, pull out the bad drive. Remove the old drive from the tray. Install the new drive onto the tray. Toss it back in. And you are done. The machine takes care of the rest.
Brilliant! The easiest drive failure change I have ever done. I am glad I have the Qnap.