OnePlus One does not connect via USB (Yes, I have tried the usual suspects)
I also have the same problem and tried whole lot of things, it did not work for me. Now only following things needed to be checked:


Found a comment on the support forum that the Marshmallow CM is having issues with USB to PC connectivity but OTG works.(I haven't tried OTG myself yet but will be definitely check it). So here we need to flash the phone to an older CM version to get the phone detected in hte PC. Reference-> https://forums.oneplus.net/threads/usb-not-working-anymore.453049 
It's an hardware issue. The USB ribbon port needs to be changed.


Also please do let us know if you got your problem solved and how.

Thanks.