I can get to the restoration screen, however, I can't restore it b/c it says the version of the OS is not valid. I tried booting it from an external drive, however, it won't recognize the OS as being valid on the external drive. I was able to get to the disk utility in restoration mode, and I tried to erase the drive, but it says it can't unmount the drive. Did all the resets of Pram and stuff but it doesn't help. I took it into the service center and the "think" it's the motherboard and suggested it would cost US$700 bux to repair Any suggestions here?