The hard drive was not meant for 24/7 use. Hard drives for regular computers were designed to take the strain of on/off operation. Running a HDD 24/7, even if it's not spinning, will cause it to fail earlier. If you want to run it 24/7, get a server hard drive. They were meant to run continuously.
there's nothing wrong with running it 24/7, it just hurts your hard drive. It will still last a long time, just not as long as it would turning it off and on.
Ex.: You leave your computer running 24/7 and the HDD only last 9,000 hours. It likely would have lasted 10,000 hours if you had shut it down. It shouldn't matter too much unless you keep a lot of files on your drive.