Hi, My 27" iMac has the infamous gray spots (not the easy condense ones), and it needs to be serviced. Before I do that, I want to wipe everything (aka format), so no personal data would be exposed. I have a regular time machine backup on an external drive. Is it possible to wipe everything out, and when it comes back, to restore everything the way it used to be? If so, please tell me what do I need to do. Thanks a lot.