I just upgraded an MC40 with Rev B hardware to KK.  I'm having a problem having my PC recognize the device and getting access to file structure.  The device and/or PC continues to want to install MTP USB drivers that continuously fail, thus resulting the PC not able to recognize the MC40 as a device.  I have toggled storage, security, and developer options and none were successful.  I also tried access through ADB.  ADB recognizes the device, but can not push files.

Rev B with JB and Rev C MC40, do not experience this problem and are immediately recognized by my PC as an MC40.