This does apply to cPanel servers only!
Stop all processes that have files open on /tmp such as MySQL and other applications using:
/usr/sbin/lsof /tmp
Run the following commands (in that order):
/bin/umount -l /tmp
/bin/umount -l /var/tmp
/bin/rm -fv /usr/tmpDSK
/scripts/securetmp
This should create a new /tmp partition for you. That's all