First thing to do is run the Disc Utility that comes with your iMac it will be in the Utitlities folder in you Applications.
Click on your hard drive in the left hand column and then click FIRST AID at the top of the right column. To see if you have any disc permission issues click verify if it shows any click repair disk permissions.
That can sort out some problems. There is a useful article on CNET
Also meant to say that while you are there verify the disk as well.
Edited by jpl1953 (Fri 27-Sep-13 15:48:47)