I have been noticing a slow down on my 2011 iMac also have had to force quit applications and reboot to stop the spinning ball. I am considering reloading but would like to verify I don't have a HDD failing. I am a PC tech and would usually just run sea tools for windows or other OEM test. I have run the short diagnostics off the second cd and first aid, both found nothing. Is there a tool that will just run a thorough test on the HDD? Free would be great but would consider paid. I do have a up to date time machine backup and an few month old image of the drive.