If your computer is on the drive will not stop spinning, unless you have told it to do so in the power settings. If it takes 15min to stop spinning you likely have the power profile set to shut off the hd after 15min.
Probably best to leave it as it is - you could change the power settings to have the disk stop spinning earlier, but this will probably put a lot more strain on the hard disk (generally there's much less mechanical strain leaving something running than starting / stopping it).
If noise is a problem you could try mounting the drive using rubber grommets - often the noise is made worse by vibrations from the drive being transferred to the case.
What Mr. Flanders is trying to portray is the fact that ALL of the responses to this problem so far has been by people who are clueless and should just STFU.All of those responses are WRONG! This problem is caused by malware and a solution is still being worked on by the experts( I am having the same problem,and that's why I found this thread).The problem has to do with the browser cache and malware replication.I will bookmark this page and come back if I find a solution.
Not true at all. First of all, you should know that you are responding to a thread that is 4.5 years old. Second, my hard drives don't stop spinning either. Why? Because that's how I set them in the power settings. While the computer is on, they won't stop spinning because of my settings. As for browser cache and malware replication? What are you smoking? It must be good...
I think the confusion is in OP's title, "won't stop spinning". Does that mean the drive actually is spinning all the time, which is normal? Or did OP mean that the Disk Activity light on the case front panel is always on. The latter meaning would indicate constant disk access leading to slow disk access and suspicion of malware activity.