I have a _posts folder that contains all my posts for my blog and projects. I'd like to separate the markdown files for my blog notes and project pages. For example, in addition to the built-in md -> HTML conversion for files in _posts, I'd like to have a _projects folder that contain my markdown files for individual project write-ups and build them into HTML when running jekyll serve.
Looks like you want to use categories. Here is a link discussing a similar problem : Multiple _posts directories