You can't play backups on the PS3 yet, the only reason you have a 500GB drive is to have more storage for music, photos and videos and stuff. He probably put the official Yellow Dog Linux on, or Gentoo or even Fedora.
When it gets to command you can type in startx after you login and it should load the user interface.
I believe to get back into the original PS3 GameOS you hold the Power Button down for about 5 seconds, I'm unsure on that because I don't have a PS3, but I'm fairly good with Linux.
Edit wait you can backup games to the hard drive in Linux but I forgot the command I know it's DD something but you can't play them at the moment because the security hasn't been breached yet.
This post has been edited by bucko: May 13 2007, 11:15 PM