Catalogue on Multiple hard drives

I have been using multiple catalogues for a while now. looking through has me convinced to go to one. I am out of space on my computer.

I want to do one Catalogue with the smart previews and catalogue data on the laptop and keep the last 3 years of images and on an external have everything from before then. And I have my most important with me...

is this something that can be done? How should I go at moving folders to my external? Or should I just do multiple catalogues?

If you can answer or point me at good instructions that that would helP. Thanks in advance.