This is defined during the Exchange install and the Unity installation documents does specify in ambiguous terms "change the defaults as per your planning on page xx"
In other words it has you work out sizing first then during install, it has a drop down for where to stick Exchange message store. If you are not careful the default will bind M to C as you have now.
I believe there is a way to move it but it is quite detailed and you should treat it very carefully. If you do not have a CCO id for gathering step by step docs then try this site:
http://www.ciscounitytools.com/ It is run by the Highest level Unity TAC engineers. They used to have an independent site called AnswerMonkey.net but then re-branded it with cisco in the title.
Lessons learned the hard way on this one. When installing Unity, the devil is very much in the details and some Exchange experience is helpful.
good luck.