[ubuntu] Using grub on Ubuntu bootup

I’ve just installed Ubuntu 19.10 on an external usb ssd. My main computer runs Ubuntu 18.04. Basically I just wanted to see if I could do it without screwing everything up. It all works fine. For most of you this is no great stretch, but I’m a novice at this and am learning through doing! Now I have the problem that grub on boot up needs to have the usb external drive plugged in or it doesn’t know what to do, and this is the problem.... neither do I. How do I boot up into 18.04 when the usb drive isn’t plugged in. When both os are available on bootup grub gives me the option to boot up the os I wish. But if the usb isn’t there then I get the grub screen prompting me and I don’t know how to begin with it. Could use some help!
thanks
Andrew