I don't know anything about your project, your version of AE, your output settings that may help us help you fix your problem. Tell us about your project, the source footage, your render settings, the version of AE you are using and the OS version for your iMac.
What Rick said - we need a lot more info. Comp settings, render settings, what version of OSX, to what location you are saving, what third-party plug-ins may be involved. All that trivial, unimportant stuff.... *lol*