Stop internal drive from loading?

So I just got my ibook back after having to send it in because of a logic board problem, no charge but inconvenienced. I got over it. Now my hard drive failed. I realize that this happens too, nothing I can do about it.

What I need some help with is a temporary solution. I have an external hard drive with an OS on it. I did that for emergencies like this, so that I can boot off the other hard drive, everything I need is backed up on it anyway. So I try to boot from it but the startup gets hung up every time when it's trying to "read local disks", it can't read it because it failed, but it still detects it and tries to load it. Anyone know of a way to stop it from loading up other disks so I can get on with my term papers, I'll get it fixed after finals but I need it now.

Thanks in advance.