When the device is set to use MTP, neither Windows, nor ADB will detect the device when connect via USB cable. I have tried multiple cables, in multiple USB ports. I have not tried other computers yet, as this is the only one that actually has the driver installed.
Disclaimer: I am running an Unofficial CyanogenMod 12 build of Android 5.0.2
Device : LG G3 (LG-D855)
Android 5.0.2 CM12-Unofficial-20141228
Google USB Driver: Installed
USB debugging: Enabled
PC: Windows 8.1 Pro Update 1
No one, except me, has reported this as an issue in the XDA thread.
I have been unable to get my PC to detect the device via USB since I first installed CM12, which was around 11/10/2014.
If i change to PTP, Windows will detect the device, as will ADB. But Windows will only let me access the internal storage, and Camera folders.
adb
on my Linux machine would not recognize my wife's G3 unless it was set to PTP mode, goofy as that is.