Available Space - incorrect

Hi - I'm having a major issue with Lightroom CC. I'm running a Macbook Pro and OS X states that I have 4.7GB of HD space available, yet when I check preferences Lightroom CC thinks that I have 54.29GB available. In spite of this, the Lightroom CC library comes to 51.82GB...!! So much for intelligent management...

I've already amended my preferences so that the photo cache is just 5% of remaining space. The size of the Lightroom CC library is now making my MacBook Pro unusable.

Any idea of what's going wrong here?

Thanks