What do editors use for project archive, at the moment I have plug in 3.5 inch HDD but I am very aware that hard drives fail. My projects are about 1TB each.
As an additional question, if I use project manager in Premiere to do a project copy what else do I need to back up to keep the dynamically linked After Effects files?
With that much data, I think your only viable solution will be hard drive.
So...make two copies for each project. (Any kind of media can fail, so making two archived copies is good practice regardless of the media you're backing up to.)
Have you seen the prices of HDDs at the moment! Does anyone use tape back-up? (LTO)