NO spaces left on device writing smstore file--throttling

HI, I'm using portable thunderbird to receive news. I'm using it in my harddisk, so, there's a lot free space in my disk.
But I encounterd an error message box reported:
A news ( NNTP) error occurred: NO spaces left on device writing smstore file--throttling.

Did you see this alert before? How can I fix this problems? thanks!