Try emptying your Recycle Bin. Also what drive do you have the actual LR program installed on and the Camera Raw cache location in Preferences> File Handling? If it's a different drive that is low on space that can cause the message you are seeing.
Thanks for your note. I have emptied the recycle bin but this still intermittently (not always but reasonably often) happens. The camera raw cache file is on my D: drive and is set to a maximum of 20GB. The program and presets folder are in the default location on the boot (C:) drive, the catalogs and previews are on the D: drive. Please note that none of my drives is remotely close to being low on space - C: has 73GB free and D: has 2TB free.
First thing to try is deleting or renaming the LR Preferences file:
You can simply rename with the extension .OLD (Lightroom 5 Preferences.agprefs.OLD), which will allow you to restore it if the problem persists.
Just my two cents.
I have a Lenovo W500 series with SSD and 140Gb free, with Windows 7 64 Enterprise.
Sometimes I have the same message in Lightroom.
I once had LR5 open in one screen, while in the secondary I had the "My Computer" explorer window. When LR warned me about the nearly full disk, I looked at the Explorer window and I saw that the C: disk was red, with less than 1 Gb free. this lasted a faw seconds, than it went back to 140Gb free.
I guess I have some glitches in the SSD (maybe it is dying, maybe Windows has a problem with the device driver), adn since Lightroom is always checking the Catalog, it catches the issue before anything else and it displays the warning.