

Be careful with powering HDDs on and off. That is actually the operation that puts the most strain on them AFAIK. Sadly there is no good rule of thumb when it does more harm than good, but I would guess if you turn it on more than once a week, you are probably doing more harm than good compared to just letting in run. Many people even intentionally turn off sleep-mode in “green” drives so that they don’t shut down automatically.
AFAIK the “wear” does not mainly come from the spinning, but from temperature changes that make parts slightly expand and contract in size. An always on HDD has pretty constant temperature.