The TrueNAS gods say to leave HDDs plugged in and spinning.

The current way to store digital data is as “living data” with backups. Data only exists if it’s constantly verified to exist. You can’t verify an unplugged HDD. Of course, you need a 3-2-1 backup system, and have to continually verify those backups as part of the living system.

The magnetic platters in a HDD are actually decently reliable. Sure a cosmic ray or error may flip a bit, but that’s why we have backups and redundancy elsewhere in the system. That flipped bit won’t actually damage the hardware.

The real concern with HDDs is that they mechanically wear out and break. They stop spinning. The motors malfunction, the lubricants dry up, the metal bearings wear down, etc.

TrueNAS says the most dangerous time in a HDDs life is when it is spinning up or slowing down from a stop. This gives the most mechanical stress on the HDD. They says it’s better to leave them powered and spinning 24/7 than to unplug them or despin them.

Keep them unplugged and in a cool dry safe environment free from temp swings or shock damage, theft, etc.

And when in use, always make sure they are on conditioned power and kept cooll while operating.

Really nothing you can do except be sensible. Don't move them while they're powered on, and keep them in a mild temperature and relative humidity environment comfortable for a human. That's about it.

Just have a backup. And a backup of your backup. That way you don't have to worry about it.

The most important thing you can do, to prolong the life of the HDDs, is to NOT do a full overwrite every month or so. Sync instead.

Do as little as possible to make the contents of the HDD an identical copy of the source.

There are many sync utilities and backup softwares you can use. I use a script and rsync.

Using a sync utility like rsync it is even possible to efficiently make versioned full snapshot backups. Then a new timestamped snapshot backup is created by comparing the previous snapshot with the original. Files that are in the previous snapshot are hardlinked from the previous snapshot, which save time and storage. Only new or modified files need to be copied.

This way it is very fast to create a updated snapshot and, as long as you have not made large changes to the original files, every new snapshot takes up very little storage.

I have two SSDs in my PC. The main SSD is used as normal with the OS, programs and user files. And a secondary SSD holding versioned rsync snapshots of the user files. Every boot a new updated snapshot is automatically created. Also old snapshots are automatically deleted as needed, so at most only 7 daily, 4 weekly and 4 monthly snapshots are retained.

I also have two multibay USB DAS. One primary DAS is used for media files, movies and so on. The primary DAS is also used for manually triggered snapshots of different folders on the main SSD. The secondary DAS is used for rsync snapshot backups of the main DAS. The primary DAS is usually always turned on with the PC. The secondary only now and then to backup the primary DAS.