Changing my startup disk

Hi,

My startup disk is currently full. I have tried deleting stuff, but that hasn't really helped. However, I can see that my startup disk has been chosen as 'disk' - whereas there is another disk called 'macbook', which has twice as much capacity (which isn't even being used!!!).

So, first of all - can I change my startup disk now?
And secondly, will the stuff on my current startup disk be deleted in the process? I hope not !!!

Thank you