hard drive works for several minutes
OS 10.8.5 iMac about 5.5 years old. The hard drive starts working several times a day for no apparent reason and goes for as much as 10-15 minutes. Sometimes it starts if I change something on the calendar. Other times I'm not sure what starts it. Sometime it even does it when it is supposed to be asleep. Any ideas or suggestions? Thanks.
iMac (27-inch Mid 2010), OS X Mountain Lion (10.8.5)