Moving .Blend files

[The Josh Machine]

I was recently working on a project when my Mac's hard drive became completely full. I bought an external hard drive, moved all my documents including my Blender project to it. I kept the original project on my Mac as well but had to move it around in the process.

Now when I open the project from my mac and render none of the materials are rendered and the scene is almost completely black. However if I open it from the external hard drive it works perfectly.

Does anyone know how I can restore the original project, and also how to avoid this issue altogether when moving projects around? It has happened before but not on such a large project, so it would be good to know what causes this for future projects.

Thanks!

Josh