Termux Android allow write external media(USB Flash SD card)

#Mount system as writable (I do this in TWRP).

#this might be /system/system/etc/permissions/ on some devices
cd /system/etc/permissions/

#backup file
cp platform.xml platform.xml.old

#modify/add these settings

    <permission name="android.permission.WRITE_EXTERNAL_STORAGE" >
        <group gid="media_rw" />
        <group gid="sdcard_rw" />
        <group gid="sdcard_r" />
        <group gid="u10_sdcard_rw" />
        <group gid="u11_sdcard_rw" />
        <group gid="u12_sdcard_rw" />
        <group gid="u13_sdcard_rw" />
        <group gid="u14_sdcard_rw" />
        <group gid="u15_sdcard_rw" />
        <group gid="u16_sdcard_rw" />
        <group gid="u17_sdcard_rw" />
        <group gid="u18_sdcard_rw" />
        <group gid="u19_sdcard_rw" />
        <group gid="u20_sdcard_rw" />
    </permission>

#reboot device
#don't access drive through '/storage'
#you must access it through '/mnt'
#example /mnt/media_rw/0000-0000


#Create Link in termux
ln -s /mnt/media_rw/0000-0000 $HOME/sdcard