I'm playing around with syncing files from my laptop to my SD card on the phone using adb-sync.
adb-sync reports successfully pushing the files to the destination.
Navigating to /storage/9016-4EF8/Music in adb shell, the new files are listed, right after the following message:
ls: ./.android_secure: Permission denied
Yet the new files don't appear in my file explorer on the laptop. Additionally, they are not seen by my music player.
adb shell (different project - see comment) otherwise lists the contents of the SD card perfectly, AFAICT. It's only the adb-sync transfers that incorrectly show up as not being present.
Perhaps this is related, but adb shell reports success when creating new files, and these files also do not appear in my file explorer (Dolphin). Creating a new file in Dolphin works fine and is correctly listed in adb shell!
So I believe they weren't actually transferred. Or they were transferred somewhere strange in internal memory. Where might they actually be?
I've had a little time to clarify my thinking about this. Actually adb-sync is not necessary to this discussion. The problem is actually that filesystem operations within the adb shell are not reflected in the view from my file manager, Dolphin. So I launched the phone's file explorer and, voila, there's the changes!
So apparently this is a problem related to the much-decried MTP protocol. I'm happy to leave it at that unless someone else has something to add.