I need to send my Mac Pro to the seller again (something needs to be checked) but obviously don't want the seller to see all my stuff. There's not a lot of things on it since I knew I'd have to send it back, but I did surf around the web, have some photos and other documents on it. And obviously, to have Mavericks installed, you have to enter your Apple ID. The Mac came with Mountain Lion pre-installed (probably by the seller) and I upgraded to Mavericks with my Apple ID. So there's probably some information left somewhere on the Mac even if I put all my files etc. to the bin. The best thing would be to erase the hard drive and reinstall Mountain Lion. I don't know if that will work, I think I need a recovery partition or something like this for it to work? (I have no install DVDs) I know that you can also do this with the internet (download the OSX basically), I'm not sure if it works on this Mac. It's a mid-2010 Mac Pro 12 core. It's a new hard drive the Mac has got so most likely the seller installed Mountain Lion on the drive himself. What do you recommend doing?