Can't Clear Minify Cache if only CSS is enabled

Hello,

I found that some of my css was stuck, so I cleared all with no luck and went into my settings for Minify, CSS is enabled but the Empty Cache button is greyed out and it’s saying Minify via disk is currently disabled. See screen;
http://i.imgur.com/lf66MLV.jpg

How can I go about clearing the CSS Minify Cache? Much appreciated.

Cheers

https://ww.wp.xz.cn/plugins/w3-total-cache/