[Android Software Development] kingkobra97:
adb push/adb pull files work fine. MTP not working in Windows too. I don't have a device tree on Github, so I'll post it here =)

BoardConfig

RECOVERY_VARIANT := twrp
RECOVERY_GRAPHICS_USE_LINELENGTH := true

SP1_NAME := "internal_sd"
SP1_BACKUP_METHOD := files
SP1_MOUNTABLE := 1

TW_INTERNAL_STORAGE_PATH := "/data/media/0"
TW_INTERNAL_STORAGE_MOUNT_POINT := "data"
TW_EXTERNAL_STORAGE_PATH := "/external_sd"
TW_EXTERNAL_STORAGE_MOUNT_POINT := "external_sd"
TW_DEFAULT_EXTERNAL_STORAGE := true
TW_FLASH_FROM_STORAGE := true

TW_NO_REBOOT_BOOTLOADER := true
BOARD_HAS_NO_MISC_PARTITION := true
TW_HAS_DOWNLOAD_MODE := true
TWHAVE_SELINUX := true

TARGET_RECOVERY_INITRC := device/samsung/3g/etc/init.rc
TARGET_RECOVERY_FSTAB := device/samsung/3g/ramdisk/twrp.fstab
TW_THEME := portrait_mdpi
BOARD_USE_CUSTOM_RECOVERY_FONT := \"roboto_10x18.h\"

TW_MTP_DEVICE := /dev/usb_mtp_gadget [COLOR="Orange"]# yes, it exists, and work[/COLOR]
TARGET_USERIMAGES_USE_F2FS := true

BOARD_UMS_LUNFILE := "/sys/devices/virtual/android_usb/android0/f_mass_storage/lun/file" [COLOR="Orange"]# exists too[/COLOR]
TARGET_USE_CUSTOM_LUN_FILE_PATH := "/sys/devices/virtual/android_usb/android0/f_mass_storage/lun/file"

init.rc

This is a stock TWRP file. I took it from bootable/recovery-twrp/etc/. Where "+", it was added/changed by me.

import /init.recovery.service.rc

on early-init

write /sys/fs/selinux/checkreqprot 0

setcon u:r:init:s0
restorecon /postinstall

start ueventd
start healthd

service set_permissive /sbin/permissive.sh
oneshot
seclabel u:r:recovery:s0

on init
export PATH /sbin:/system/bin
export LD_LIBRARY_PATH /sbin

export ANDROID_ROOT /system
export ANDROID_DATA /data
[COLOR="SeaGreen"]+[/COLOR] export EXTERNAL_STORAGE /external_sd [COLOR="Orange"]#was /sdcard[/COLOR]

mkdir /boot
mkdir /recovery
mkdir /system
mkdir /data
mkdir /cache
mkdir /sideload
mount tmpfs tmpfs /tmp

chown root shell /tmp
chmod 0775 /tmp

write /proc/sys/kernel/panic_on_oops 1
write /proc/sys/vm/max_map_count 1000000

on fs
mount pstore pstore /sys/fs/pstore

[COLOR="SeaGreen"]+[/COLOR]on boot
[COLOR="SeaGreen"]+[/COLOR] write /sys/class/android_usb/android0/iManufacturer Samsung
[COLOR="SeaGreen"]+[/COLOR] write /sys/class/android_usb/android0/iProduct $ro.product.model
[COLOR="SeaGreen"]+[/COLOR] write /sys/class/android_usb/android0/iSerial $ro.serialno
[COLOR="SeaGreen"]+[/COLOR] write /sys/class/android_usb/android0/f_rndis/manufacturer Samsung
[COLOR="SeaGreen"]+[/COLOR] write /sys/class/android_usb/android0/f_rndis/vendorID 04E8
[COLOR="SeaGreen"]+[/COLOR] write /sys/class/android_usb/android0/f_rndis/wceis 1

[COLOR="SeaGreen"]+[/COLOR] write /sys/class/android_usb/android0/enable 0
[COLOR="SeaGreen"]+[/COLOR] write /sys/class/android_usb/android0/idVendor 04E8
[COLOR="SeaGreen"]+[/COLOR] write /sys/class/android_usb/android0/idProduct 6860
[COLOR="SeaGreen"]+[/COLOR] write /sys/class/android_usb/android0/functions mtp,adb
[COLOR="SeaGreen"]+[/COLOR] write /sys/class/android_usb/android0/enable 1
[COLOR="SeaGreen"]+[/COLOR] setprop sys.usb.state mtp,adb

mkdir /dev/usb-ffs 0770 shell shell
mkdir /dev/usb-ffs/adb 0770 shell shell
mount functionfs adb /dev/usb-ffs/adb uid=2000,gid=2000

on boot
ifup lo
hostname localhost
domainname localdomain

class_start default

# Load properties, pre-Android 6.0
on load_all_props_action
load_all_props

# Load properties, Android 6.0+
on load_system_props_action
load_system_props

# Load properties, Android 6.0+, vendor init lives here
on load_persist_props_action
load_persist_props

on firmware_mounts_complete
rm /dev/.booting

# Mount filesystems and start core system services.
on late-init
trigger early-fs
trigger fs
trigger post-fs
trigger post-fs-data

# Load properties, pre-Android 6.0
trigger load_all_props_action

# Load properties from /system/ + /factory after fs mount. Place
# this in another action so that the load will be scheduled after the prior
# issued fs triggers have completed.
trigger load_system_props_action

# Load properties, Android 6.0+, vendor init lives here
trigger load_persist_props_action

# Remove a file to wake up anything waiting for firmware
trigger firmware_mounts_complete

trigger early-boot
trigger boot

on property:sys.powerctl=*
powerctl ${sys.powerctl}

service ueventd /sbin/ueventd
critical
seclabel u:r:ueventd:s0

service healthd /sbin/healthd -r
critical
seclabel u:r:healthd:s0

service adbd /sbin/adbd --root_seclabel=u:r:su:s0 --device_banner=recovery
disabled
socket adbd stream 660 system system
seclabel u:r:adbd:s0

# Always start adbd on userdebug and eng builds
on property:ro.debuggable=1
#write /sys/class/android_usb/android0/enable 1
#start adbd
setprop service.adb.root 1

# Restart adbd so it can run as root
on property:service.adb.root=1
write /sys/class/android_usb/android0/enable 0
restart adbd
write /sys/class/android_usb/android0/enable 1

fstab

/external_sd vfat /dev/block/mmcblk1p1 /dev/block/mmcblk1 flags=display="Micro SD";storage;wipeingui;removable
/data ext4 /dev/block/mmcblk0p25 default wait,length=-16384
....

When I want to open some file from my external SD, I receive this error