I was updating my Xbmc machine, which we use to play videos recorded by MythTV. Running apt-get kept complaining that it was out of disk space, yet ‘df’ showed there was plenty on every partition.
Turns out that’s because it’s lying. It’s not out of disk space, it’s out of inodes. For some reason it’s not cleaning up old kernels, and is leaving the kernel headers in /usr/src. That’s a staggering number of tiny files that eat up inodes much faster than they eat up disk space.
So, if you see this error, you need to remove old kernels and headers to clean up space.