Android OS Tips for Marshmallow and Nougat
The first time you plug your new device into your PC, you will have to allow access. Make sure to choose to allow for full functionality of any USB sync.
Once plugged into the PC, the USB mode should default to "USB transferring media files" which is the correct mode for USB Mass Storage mode sync.
Android 6.0 Marshmellow
For Android 6.0 Marshmallow users, the USB connection mode may need to be changed to "Transferring Media Files" because the connection mode defaults to "Charging."
You may be prompted with a message that says "Allow access to device data" when you plug in your phone. Tap "Allow" on this message to sync if you see it; this ensures you're using the correct USB connection mode.
If you aren't prompted with "Allow access to device data," pull down the overlay menu by dragging your finger from the top of the screen down, then locate a message that says "Connected as charging" or "USB charging." Tap that, then look at your options -- you want to select either File Transfer or Media Transfer. Once you do that, wait about 30 seconds and the sync should start on the PC.