You may be having a problem where the disk drive is disconnecting after a certain period of time, especially in Windows 8 or Windows Server 2012. These new operating systems have implemented more aggressive power management profiles. While good for reducing energy consumption, some of these settings interfere with the external hard drive and reduce power availability to it, thereby making the hard drive unable to spin.

However, if the disconnections seem to happen at regular intervals (every 8 to 15 minutes) you may be experiencing a different issue relating to Windows changes in the USBSTOR.sys driver. There is a hotfix available from Microsoft at 
http://support.microsoft.com/kb/2919355 that will resolve the issue. If after applying this hotfix the issue is not resolved, please contact DataLocker Support, as there is a registry patch available (we cannot be responsible for any damage to your computer from changes made to the registry! The patch has been used by customers several times with no issues but we recommend creating a system restore point before applying any registry changes).

In Windows 8
Open Power Options by clicking the [Start] button, click [Control Panel] in apps, click [System and Security], and then click [Power Options].
Under the plan that you want to change, click [Change plan settings].
On the Change settings for the plan page, choose the display and sleep settings that you want to use when your computer is running on battery (if applicable) and when it's plugged in.

In Windows Server 2012
To configure this using a GUI, go to the Start Menu, search for "Choose a Power Plan" under Settings, then select the "High Performance" option.
To configure this from a command line, use "POWERCFG.EXE /S SCHEME_MIN".