Depending on your host operating system and usage patterns, you may wish to change your computer's the default behavior regarding spinning down of disks. Because of modern low-friction bearings and lubricants, once you get the disk spinning up to speed, it takes very little energy to keep it spinning. However, after you turn the disk off and let it spin down, it requires quite a bit of energy to rapidly bring it back up to operating velocity, and puts the disk under strain of acceleration. This strain can shorten the disk's life expectancy, costs power, and gives you a performance hit as the disk must be spun up before access. 

When to enable spindown

A disk that is not used in day to day operation may be set to sleep at idle. This is a  convenient substitute for turning off the disconnecting the disk. 

When to disable spindown

A disk that is used frequently should not be set to sleep, as the constant start/stop cycles  will greatly strain the mechanism and shorten its service life. Also, some computers and Operating system combinations have trouble accessing disks after issuing wake up commands.

How to access disk sleep control

Windows 7

Access the Power Options by clicking the Start button, clicking Control Panel, clicking System and Security, and then clicking Power Options.

  1. On the Select a power plan page, click Change plan settings next to the selected plan.

  2. On the Change settings for the plan page, click Change advanced power settings.

  3. On the Advanced settings tab, double-click Hard disk. Under the "Turn off hard disk after" Setting, select "Never" to turn off disk idle sleep, or a value in minutes to enable.