I've got a server that is very low on disk space, on the C: drive. It's a critical server, that we're working on replacing, but still need to rely on it. In order to free up disk space, I've rebooted it in an effort to get a couple 100 MB back. Normally this works, but today when I attempt to do it, for some stupid reason it is installing some update (I have no idea which one, how large, etc). I'm concerned that there might not be enough disk space on C: to install this update, that it will sit there for hours, perhap says, attempting to install this update, etc. I'd like to know, as a rule, do updates that insist upon being installed, always make certain that there's enough disk space to install, or do they totally ignore irrelevant things like making certain there's enought free disk space so they can be applied?
From my experience security updates do not check for disk space.
I think the root problem though is that an update is being installed that you didn't know about. As a general rule, no critical server should be applying updates on it's own. These servers should be set to check for updates only and the admin should be testing the updates on another server if possible or at least looking at the updates and applying them manually. If you're that low on disk space I'd stop updating that server completely and instead work on replacing it as quickly as possible.
To free up some disk space you might want to look at reducing the page file size but be careful as this might impact performance. Or move the page file to another drive.