Hmm, that's weird. I have never seen XboxHDM say "no space" on C drive. Maybe it's a little bug Kingroach has to work out. I know the backup C drive option doesn't work, at least with my build, and he said he was going to be fixing that and some other things. I'll let Kingroach handle this question, because I've never seen this and have no idea what it could be.
As for not being able to write files to the C drive after the softmod, that is a safety feature. If you want to write files to the C drive, launch Ndure Toolset.
As for uninstalling the softmod and going back to normal, there are a couple ways to do that. If you made a full backup of your C drive before installing the softmod (probably not, because I didn't tell you to) then you can restore that using either Ndure Toolset or XboxHDM. The other way would be using NDURE toolset and FTPing in, deleting everythign in the C drive and FTPing over a stock C.
NOTE: Kingroach, I tried restoring the C drive from SlaYer's and got a few errors halfway through, not sure if this was known or not, but it was really weird.
By the way, to all the people asking if you can use this tutorial to install on a completely fresh, brand-new hard drive: I tried today installing on a freshly formatted hard drive and I couldn't get it to work. No matter what I tried, nothing was working without the stock C files already on the drive.
SIDENOTE REQUEST: HSD gave me edit abilities, so that I could maintain this tutorial, and I plan to do so. But I have a question for all you smart people. When I initially posted the tutorial, I posted it as three posts, one right after another. It combined these three posts into one post for some reason, not sure why. Well, that's fine, but now I can't edit it. Whenever I go to edit the post, and hit submit, it says there are too many images (which there are, that's why I wanted it to be 3 posts). So how can I edit my tutorial?