What's the difference from an external hard drive and a external SSD, I want one of these so I can back up from my iMac just in case it fails or corrupts. What is time machine and how does it work? Could a SSD be just as good and do the same job as in keeping my stuff on a hard drive just in case my iMac fails? I just want to know what's better but will keep my information on an external device just in case my iMac fails. I rather be safe than sorry. Thanks for all responses!