Boot on external HD

Hi, I have a unibody macbook 2.4ghz with 4gb of RAM, I'm running 10.8.2 fairly smoothly on it but have experienced issues booting on my external HD that contains 2 partitions (my old system back up) 1st partition runs 10.6.8 and 2nd partition runs 10.5.8
The partitions mount fine so that's not the problem.
My problem is booting on them to use my printer, I used to be able to boot without any issues before but since the upgrade to Lion, it's not happening (it worked once or twice, after multiple tries), I NEED to boot on my 10.5.8 to use all the features of my expensive Xerox Phaser 7400 printer (which works under LION, but NOT with all the features (like feed from different tray and output to the left side rather than the default Top output...).
I thought of buying a new laptop but then I would have to say goodbye to using my printer...
I'm stuck and can't seem to be able to switch back to my previous system nor boot on it
Please help me !!!
Thank you

I'm assuming you mean you upgraded to Mountain Lion, not Lion as you state above that you're using 10.8.2.

The ideal solution is to inquire of Xerox to see if they have an improved driver for your Phaser 7400 that supports all the features you need. If Xerox does not have improved drivers, then I guess you need to go through all the gyrations of booting to different versions of OS X.

The upgrade to Mountain Lion should not have had any effect on your external hard drive as that would only change things on your internal drive. That tells us there may be something wrong with the external drive. All I can suggest for now is run a verify - repair of the external hard drive using Disk Utility from Mountain Lion. If that doesn't help, reinstall Leopard and or Snow Leopard to the external drive. And if that doesn't work, you probably need a new external hard drive.

And - a last suggestion: Instead of dividing an external hard drive into two partitions which can always cause problems, buy two external hard drives or two flash drives of at least 16 GB each and separately install Snow Leopard and Leopard on them. Then you can boot to either with ease.