How to retain folders from External drive on the sidebar?

I dragged some folders from my external hard drive that I regularly use when I am in the office.

Sometimes I take my MBP out without the external drives. I realize that these aliases I had created on the sidebar disappears.

When I connect up to my external drive again, I have to drag these same folders again into the sidebar.

Is there a way I can keep these external folders on the sidebar, without creating a real alias on my hard drive and copying that onto the sidebar?

Thanks!