I need to back up my Windows 7 computer. For the data, I just copy and then paste into an external drive. I recently read that I should do an Image Backup and in researching I found a site (Bob Rankin) that suggested Macrium Reflect. Well, I really know nothing about how to do this, but know I have to try and so I would like to know what size of external drive do I need to do this backup? Thank you.