I tend to keep a lot of stuff on my hard drive. Modern drives are big, and modern filesystems don't have a problem with searching through long, fragmented free lists that made the old suggestion of "keep the disks less than 90% full" smart. I defrag occasionally, and (at least on my laptop) a high-speed SD card configured for Readyboost to improve application-launch induced disk seeks.
So I've been getting popups (no, not malware) for several months reporting that I'm running out of disk space. These are Windows-looking officialish "Warning Event Notification" popups, reporting that "disk free space has fallen below the configured threshold." Annoying, displays in the center of the screen (even when locked/logged off) and takes focus from my work.
It turns out this particular message is caused by the Dell OpenManage Client utility that the company uses to set the BIOS password for the system, and it's controlled by a registry key: HKLM\SOFTWARE\Dell\OpenManage\Client\SysInfo\HDDThresholdValue. I set it to 0 to get rid of the messages entirely.