When should you turn off an external hard disk?
There is no one right answer and a number of issues should be considered.
Here's an interesting question: should I turn off my external hard drive when not in use? Leo Notenboom, who I , addressed this issue a few days ago on his Ask-Leo.com Web site.
In short, there is no one clear answer, a number of issues have to be considered. What surprised me, though, about Leo's answer, was that he didn't mention my reason for turning off my external hard disk.
Being a pessimist is necessary for defensive computing. The main reason I turn off my external hard disk is to protect it from me.
When my computer boots, the first thing it does is run a scheduled backup of my important files to the external hard disk. After I verify the backup ran successfully, I turn off the hard disk and leave it off until the next backup. The disk is used exclusively for backup. My backups are run by a .bat file and, in case I forget, the last thing it does is remind me to turn off the hard disk.
Why? With the disk off, I can't delete files by mistake. And, should some malicious software make its way onto the computer, it can't screw up files it doesn't see. Finally, hard disks run hot (I've seen up to 140 degrees Fahrenheit) and while there are, no doubt, air vents in every enclosure, it's only a matter of time until dust clogs them up. An external hard disk that runs for only a few minutes a day will never get dangerously hot, even if all the air vents are clogged.
But that's me. Read and decide for yourself.
I'll contact some hard disk vendors and point them to this posting. If any of them respond with a recommendation I'll let you know.