Let me start by saying "Thank you" to Linken as I was unable to play Gravemind until I saw his solution:

"In your Halo 2\maps directory, make a copy of "07a_highcharity.map" and name it "07a_highcarity.map". Don't rename or replace the existing file, you'll need BOTH files present, one with the typo and one without."

I found that as long as I didn't re-boot my PC or turn it off, I could resume from my latest Gravemind checkpoint. But as soon as I re-booted or turned off my PC and then tried to load Halo 2 and resume my latest Gravemind checkpoint, I would get the 'Insert original Halo 2 CD" error message again.

My solution to this was to quit Halo 2, delete the "07a_highcarity.map" map that I created before and redo Linken's solution.

Then I loaded Halo 2, selected Campaign and then selected Resume and voila! my latest Gravemind checkpoint was alive and well.

I hope this works for you as it was a pain in the butt having to finish this level in one sitting before I tried this.

Cheers.