Time Machine and ext hd back up

I currently have an iMac and for my iMovie files, I save all of the clips to an external hd when I importing them into iMovie. I also save archive files to a separate external hd.

With that being said, I was messing around with Time Machine last night and I noticed that I could back up external hard drive to yet another separate ext hd that I use with Time Machine. Am I correct in this observation or does Time Machine not back anything up except what's on your iMac or other Apple product?

If it does allow back ups of the external hd that I use to store the imported iMovie videos and the archive videos, what happens if say the hd that I store all of my iMovie imported clips dies? How would I go about restoring that external hd with the Time Machine back up?

I guess I'm asking because I have not backed up any of the imported video clips in iMovie. I have another external hd that I use to archive the video clips and I have a 3rd external hd that use as a back up archive of the video clips. My thought process is, if the movie clips hd dies, I can always use one of the archive ext hd and just simply re-import the video clips into iMovie. I'm just looking to streamline my workflow. If Time Machine does allow me to back up these ext hd, then I'll now have a back up of my imported iMovie files and of my archive files. This would make life a little easier.