Canvas size - defaults

Hello

I recently had to reinstall Photoshop (thanks to an Adobe expert screwing up the settings then he left the chat!)

I am looking to change my default CANVAS SIZE from Pixels to CM.  I have already got the preferences / units set to CM and when I change the IMAGE SIZE its automatically coming up as CM, however when I go into CANVAS SIZE its always pixels..  I have no idea how to get it back to CM.

Its a weird one, no idea how it happen or how I go about getting it back.

Also my scratch disc options are no longer allowing me to add a second scratch disc - any one able to help on this please?

Thanks 

Dave