The kids got bit by this the last few days after an upgrade to Android 6. It’s a known bug in Android that existed before the release of 6. Fix below.The problem is that when you try looking at media files on the phone over a USB connection many/most of them are not visible.
The solution is to force the Android Media Manager needs to rebuild its indexes for all media on the phone, including any on an external SD Card.
You may find other instructions online on how to do that, with slightly different navigation.
This is what worked on our stock Moto G 2nd Gen handsets now running Android 6:
1. Go to “Settings”… “Apps”.
2. In the upper right click the menu with 3 vertical dots.
3. Select “Show System”.
4. Scroll down to “Media Storage” and select.
5. Click “Force Stop”.
6. Click “Storage” and then press “Clear Data”.
7. Power off the phone and then on again.
Once the phone boots back up all media should have been re-indexed and now visible over a USB connection.