This is often due to the operating system putting the drives to sleep. If you do not want the drives to spin down, you will need to turn off the sleep settings in your associated operating system.
MAC OS X Systems
In MAC, go to System Preferences --> Energy Saver --> You will see an option to "Put the hard disk(s) to sleep when possible" Click this box to enable the feature.
Windows XP Systems
Go to Control Panels --> Power Options --> Turn off hard disks. There will be a pull down window where you can specify a specific amount of time the computer can be idle before the hard disks are put to sleep.
NOTE: if you are a non-linear video editor, you may run into problems putting drives to sleep to quickly. We reccomend not putting video drives to sleep.