Good thread idea.
I found if you have videos copied to your internal HDD (that show up on your XMB video column), and your open mmCM and link vids to showtime, they get duplicated.
Remove the dupes from mmCM (XMB view is best) to regain all that space.
I gained +60gb after realizing this.
Also if you store PKG files and update pups on your internal (so you can quick swap FW), you can always back them up to your external drive to conserve space archiving apps and updates.