If you happen to use ext as your default filesystem, check the output of tune2fs; it's possible your distro has conveniently defaulted some 2-5% of disk space a

If you happen to use ext as your default filesystem, check the output of tune2fs; it's possible your distro has conveniently defaulted some 2-5% of disk space as "reserved" for just such an occasion. As the root user, in a pinch, you can set that to 0% and immediately relieve filesystem pressure, buying you a little bit more time to troubleshoot whatever the real problem is that filled the disk in the first place.