Hard link
From OS X Scientific Computing
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Hard Link on HFS+
The Mac OS X default file system in HFS+. Most of unix behaves normally on this. Hard links to do not.
If you have a hard link to a file, and you replace the original file with a new one, or move the original file and then move it back, this breaks the link. So be careful.
Things can get even worse, as documented by the RixStep snarky guy.
