Reusing an old harddrive as a secondary storage on a new computer, want to know what I can delete from it

Basically the title. It was my old laptop's hard drive, and now I want to use it as extra storage on my shiny new desktop, but there's a bunch of the old necessary stuff from it that I know doesn't need to be there anymore. The only resources I've found online are how to completely format it and get rid of everything, but I want to keep all of my old data. Is there an app I can use that would get rid of everything but my downloads, documents, game saves, etc?