1. Macs have very separate user directories and happily boot off external devices.
2. Is that really necessary if it's a hardware problem?
Punter: Computer turns on but no display. Symptoms concur with notice buried^Wposted on Apple website which says that the video card is fried. Your website has tested my Mac's serial number and said it could be in an affected batch.
Insufferable genius: I will deign to say that you, the customer, may (but only may) be right in this case. Please fill this form in on my shiny iPad.
Insufferable genius' tablet: User password please.