I was recently doing an export for a pretty big MOSS site and I ran across the following error while using the following stsadm command:
stsadm -o export -url http://servername/site -filename D:\backup\sharepoint.cab -includeusersecurity -overwrite
We verified that the C drive had 1 GB of space and the D drive had 50 GB of space. Simple enought right. Wrong!
After some head scratching, we figured out that the export uses the C drive to store the entirety of the backup files before writing it to the D drive. More specifically, the location pointed to by the TEMP environment variable was being used for the backup.
The solution we found was to change the TEMP and TMP environment variable to point to a temporary folder on the D drive and then do the backup. (I had to also logout and login in order for the variables to take effect).
detail step to change the temp is here