Moto X4 running Android 8.1.0. Attempts to connect the phone to any of my Windows 10 PC fail. The phone begins to charge but under Settings, USB says Not Connected. The phone does not appear in Device Manager on the PC. A Moto E4 running Android 7.x works without issue, allowing me to see files on each internal and SD.
There are reports that the update to 8.1 has disabled USB storage functionality but there is a fix on the way (time table unknown). The only work around right now is to disable your lock screen, reboot, then you can connect it. This worked for me but it will stop working again as soon as you re-enable your lock screen (which obviously everyone should).
Sorry, but long story short, the update broke it and you'll have to wait for a fix.
A Motorola agent confirmed the issue on the official Lenovo forum