Switched start up disk

I think I screwed up. I have an older macpro (2010) that has a couple of hard drives. The start up disk has os10.13.6 on it. I have another drive with an older OS on it and I wanted to start the computer with the older OS to open some old programs. I went Into disk utility and selected the older drive as the start up disk. Hit the restart with the old start up disk selected and now the computer wont restart. It boots to the apple logo and then shuts down. I have tried holding shift key and also tried holding command+R keys and it still does the same thing. Any other suggestions to get it to boot up so I can switch back to the newer HD?