You have to mount hda6. Use the mount command in a console window to see what is mounted on your system. If /dev/hda6 is not there then as root mount /dev/hda6 /mnt/hda6 will mount and now anything you save to /mnt/hda6 will be on the device hda6. If you want to use the space all the time you should modify the /etc/fstab to mount /dev/hda6 on boot.
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ie. remove the # from the /dev/hda6 line that should already be there.
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