I remember the Gamecube had a bunch of ports on it that I was never very clear what they were for as well. I knew about all the other stuff but had no idea there was a space inside of that thing big enough for an entire full-size hard drive. I would be willing to bet that not many people utilized this feature.

I had both the FAT and slim version. The FAT was stolen and the slim version suffered from disks scratching on the tray while rotating and this eventually made the games unplayable. I don't know what happened to the slim one, i probably gave it away to someone :)