SD card as USB drive -root- 1.2 Apk

Only for some devices. You have to know if


su
setprop persist.sys.usb.config mass_storage,adb
echo DEVICE-POINT > LUN-FILE
exit


or


su
setprop persist.sys.usb.config mtp,adb
echo "" > LUN-FILE
vold
exit


works for your device.

* requires root privileges
* does'nt require BusyBox

1. when installing the widget on the homescreen confirm superuser rights
2. widget trys to find the LUN-file and the USB-device node
3. configure LUN-file and device-node
4. press backbutton
5. widget should show the current mode (USB or MTP)
6. tapping the widget should change the current mode
7. connect with USB-cable and enjoy ( hopefully )
8. disconnect USB-cable
9. switch back to MTP (otherwise, some apps may not work)

As USB-drive only mounts the external SD-card. With MTP are internal and external memory available.

If you got trouble read this thread. A lot to read but interesting.

https://forum.xda-developers.com/showthread.php?t=2297888

positive list:
Samsung S4 GT-I 9505, 4.4.2. Kitkat Touchwiz
/sys/devices/platform/msm_hsusb/gadget/lun0/file
/dev/block/vold/179:33

to complete the P O S I T I V E - list please review or email with:
+ phone type
+ Android version
+ lun file path
+ device node

Thanks to Mohammad Abu-Garbeyyeh for the app
https://play.google.com/store/apps/details?id=com.mohammadag.samsungusbmassstorageenabler
and Senior XDA Member MetaIIica for his helpful posts.