I'm currently looking at the Power on hours, power cycles and Start/Stop counts for my HDDs.
The drive in my desktop is set to power down whenever possible (since bought 5/6 years ago) and has had a default setting of after one minute for about 2 years. In that 2 years, start/stop counts approx 39,000 have overtaken the power on hours approx 12,000, and the the power cycles approx 11,000.
Currently I'm checking how many power cycles, start/stop counts and power on hours occur during an hour then a two hour period (in actual time). This will tell me how many times it powers on/off just browsing the net (not watching YT vids etc), and typing in MS Word etc - but as well as this, I wish to learn if a Power On hour means an hour for which power is supplied to the drive or an hour the drive is actually spinning. I will do another test with the computer in sleep/standby (but not hibernation/safesleep) in order to see if POH also increases when the computer is in sleep mode.
If you do something similar to me, you might be able to use this to decide whether to have your hdds power down. Download a SMART utility, one I really like in Windows is 'Crystal disk info' which is free, just bing it, and on Mac, SMART Utility, or SMARTReporter are the ones I have used. Crystal also allows you to get the smart status of external drives over USB (only disks not pen drives) - i found out that the old 6GB Quantum Fireball from and 1998 iMac had 8,900 hours and only a few hundred / 1000 power cycles. All my HDDS have been reported as healthy, apart from a 2007 Seagate laptop hdd i knew was dying.
I think its worthy of note that the I often hear the drive power down and up, sometimes the down is followed immediately by an up (but it always spins all the way down before spinning up).