Quote:
cat: write error: No space left on device


That's expected. The command that Mark gave you simply reads the content of /dev/zero and writes it to /dev/hda3. Since /dev/zero is an infinitely-long sequence of zeros, it'll stop when /dev/hda3 is full, giving that error.

And /dev/hda3 is only 16Mb in size, so it'll fill up quite quickly.
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-- roger