Motorola Nexus 6 won't show its filesystem I've had a Motorola Nexus 6 for some time now, and I tend to be a bit static with it -- I'd uploaded the majority of my music library at the time and then didn't change much for a long time.

Now I recently purchased a number of new songs and wanted to upload them to my phone, but when I plug it into a computer (any computer I've tried so far, two linux systems and one windows 10) they recognize the Nexus 6, but when I try to open it in a file manager they only show an empty folder. Rebooting either the computer or the phone had no effect.

When I attempt the same thing on my Samsung Galaxy tablet, the tablet prompts me to enable sharing and after I confirm that I can browse the filesystem on my tablet, but the Nexus 6 phone never shows me that dialog.

It's entirely possible that I accidentally enabled a security feature and forgot about it; can anyone offer suggestions where I should start looking?
It took a lot more searching than I liked, but this question put me on the right track. 

What you need to do is connect via USB, then unlock the phone and drag the top bar down. One of the notification areas will say "USB charging this device", and when you tap it you're offered the choice of USB charging, MTP file sharing, PTP photo sharing, or MIDI passthrough. Selecting MTP filesharing will let your computer browse your phone's files.