Hello. My sister has gave me a external HD that I will use to store some programs and apps. This disc has been used with Windows for some time. It is there any chance, if it's infected with viruses or trojans that it can affect my iMac? Is there way to get 100% sure that it's clean? Any app? Or it is unnecessary? I know that there are now viruses for Mac, just some trojans. But I want to be sure.