Blackberry Priv won't connect to Windows 7 for file transfer
This was a hardware issue rather than a software issue. I had the same problem with two different Privs, so I thought it must be something to do with the Android system. But it turns out that, for some reason, my phone didn't like using a USB 3.0 port. That only charges it. Switching to a USB 2.0 port finally got me the charging notification, so I was able to switch. As a side note, setting it to "File transfers" (MTP) didn't work. I had to use "Photo transfer (PTP)" to get it to see the device. And in that case, it only sees the onboard storage, not the memory card. So it's an imperfect solution, but since my photos are on the phone storage, not the card, and I can copy from the card to the phone if I really need to, it works well enough to solve my issue.