I connect my phone to a windows-PC using a USB cable. I updated to Android 6.0 and chose “external storage”, when prompted during installation. From my windows-PC I can open a folder/device named FP2 but it contains no files or directories. Before the update, it looked similar when the phone´s screen was locked. Now it´s irrelevant if the screen is locked, I can never see any files or directories on the SD card. When I access the SD-card from the phone, I can see all files and directories I expect. Why can´t I access the SD-Card from the PC?
I would like to add, that I also can´t access the phone´s internal storage vie the USB cable from the PC. For example, I can´t transfer camera images from the phone to the PC, because I don´t see any files saved on the phone in Windows Explorer.
I didn´t know there is such a thing like MTP mode, maybe I´ve done that with 5.1 once, but forgot about it because the selection was permanent. When I switched into it like Chris52 suggested, I was able to access all files.
So, this was obviously a case of operator´s fault, but the menu was way easier to find, when it popped up in the middle of the screen. Considering, that the phone keeps displaying a message “SD card connected” permanently, I tend to ignore messages in the uppermost line of the display…
In the Settings -> Developer Options* there is an entry “Set USB configuration” (or similar, in german localization settings “USB Konfiguration auswählen”). There you can set the default setting for the USB connection mode, like charge only, MTP mode, PTP mode, and more.
*) To enable Developer Options, tap Settings -> About phone about 9 times or so