I have a disk for local backups (that is the only purpose of that disk). I was wondering what would make it last longer:
Keep it mounted to my server permanently (current solution)
Keep it unmounted most of the time, mount it when I'm going to do a backup (either daily or every 3 days, I don't mind changing that) and unmounting after the backup is done.
Mounting or unmounting a filesystem won't make a difference for drive longevity.
If you want to keep your backups secure, you want to keep them offline, so if you get ransomware it doesn't encrypt your backup too. (Or if you just mistype a command and target the wrong device, folder, etc.)
But drive motor starts and stops are when the most failures occur. So the ultimate question isn't how to make a drive last longer, it's how you plan to handle it when the failure inevitably occurs.
I recently bought a second external drive to do a backup of the first one. In the process I'm going to switch to btrfs. It can do data scrubbing which allows for self repair of corrupted data, which can occur if you leave a drive unpowered in a closet for some years.