MacBook keeps booting into system installation after failed installation

I did try to install Mavericks from the original installer package onto a separate disk via Firewire. The machine used is a MacBook Pro which is newer than the Mavericks package. As usual, after starting the installation from the desktop, the system rebooted to perform the installation after a minute or so. After rebooting it got stuck on a grey screen with a slashed circle on it. I did remember that Apple some when in the past introduced this nice feature to prevent installing an older MacOS on a newer Mac, so I booted into the recovery mode and installed on the external disk from there. So fine, so good - but when I unplugged the external disk to reboot into my normal system again, I ended up on the grey awful screen again - looks like the Mac insists to continue the failed installation which is not possible. Using the option key on booting to select my normal boot partition does not work, it does not show up. So I assume the installer has put something to mark this system as installing. I don't want to reinstall my entire system to get rid of it for several reasons. So now comes the question: What did it put there? I can handle a bash to clean it out manually, if I just knew what and where.

Thanks in advance, Peter