title: 'Series' objects are mutable, thus they cannot be hashed error calling to_csv
body: <p>I have a large Dataframe (5 days with one value per second, several columns) of which I'd like to save 2 columns in a csv file with python pandas df.to_csv module.</p>
  
  <p>I tried different ways but always get the error message:</p>
  
  <p>'Series' objects are mutable, thus they cannot be hashed</p>
  
  <p>which I found a solution for in connection with groupby but not with filesaving.
  Somebody has an idea for me?</p>
  
  <p>Here a part of my Dataframe:</p>
  
  <pre><code>DateTime
  2015-07-14 00:00:00    414.37
  2015-07-14 00:00:00    414.37
  2015-07-14 00:00:01    414.29
  2015-07-14 00:00:02    414.14
  2015-07-14 00:00:03    414.21
  2015-07-14 00:00:04    414.05
  2015-07-14 00:00:05    414.05
  2015-07-14 00:00:06     414.2
  2015-07-14 00:00:07    414.54
  2015-07-14 00:00:08    414.39
  Name: CO2abs, dtype: object DateTime
  </code></pre>
  
  <p>Edit:
  sorry - forgot the code...</p>
  
  <pre><code>df.to_csv('alldatcorr.csv',sep='\t',cols=(df.CO2abs,df.CO2corr))
  </code></pre>