Help with Lion instal please

MacMini on 10.6.8. Hi I downloaded Lion but was unable to instal because the instal button on the download was greyed out. There is 35gb free space. I contacted Support but after some time they were unable to help! I persevered & got it to instal on a separate hard drive. Lion works fine when I start up with this secondary HD but of course it has not merged with all my data etc. If I restart without the HD plugged in everything is as it was on Snow Leopard inc the Lion instal app. As before if I try to instal, after agreeing terms etc it shows my Macmini HD as where to instal BUT it states "This disc cannot be used to start up your computer" and the "instal" button remains greyed out. Is there a way to resolve this either by transferring the Lion from the secondary HD or by sorting this problem on the original HD?
I am not a proficient IT man, so if you think you can help it will be very much appreciated but I may need fairly concise help I'm afraid.

Thanks