title: git problem : fatal: Unable to write new index file
body: <p>I have an existing project that has been managed under git. I recently had to do a ground-up rebuild of the computer (OS X Snow Leopard), and returning to the project, git can track the changes in the project, but I can't save anything to git. I get the error in the title when I try :</p>
  
  <pre><code>Dans-iMac-335:[app-name-obscured] apple$ git add app/models/*
  fatal: Unable to write new index file
  </code></pre>
  
  <p>What's the best way to recover from this situation? There is a heroku repository version of this, but it is very out of sync with this one. presumably I could use a new clone as a basis, and update each file, but that seems over laborious and error prone. </p>
  
  <p>What is git looking for here that it can't find, and is there a change that will restore it?</p>
  
  <p>BTW, file permissions are correct from what I can tell - the files under control and the .git components are both owned by me with rw access...</p>