You have to install a third party Android File Management app, such as ES File Explorer, File Browser, etc..
In this instruction, we're using the ES File Explorer and LG NEXUS 5X smartphone.
***please note, the external storage device must be in FAT32 format****
The ES File Explorer app is a free app from Google Play Store, https://play.google.com/store/apps/details?id=com.estrongs.android.pop&hl=en
1.) Plug in the 3-in-1 adapter to smartphone, insert a memory card or USB flash drive to adapter and launch the ES File Explorer app.
2.) In this instruction, we're using the Samsung Micro SD card with 128GB capacity. As you can see the app detects the external storage device, USB1003.
3.) Tab " Internal Storage> " and select DCIM, it is the place where the photos taken from smartphone is stored.
4.) Tab "Camera" under the DCIM folder.
5.) Inside the Camera folder, you should see all the photos. Tab and hold the selected photo will give you circle checked dot.
6.) Mark the checked dot on the photos you wanted to transfer.
7.) Tab the "COPY" at the lower task bar menu.
8.) After that, tab the Backward key to go all the way back to Home menu and enter into the external storage device.
9.) You will see a root folder "UNTITLED", enter into the folder.
10.) Tab the "+'' sign to create a new folder
11.) In this instruction, a Test folder is created.
12.) Go into the Test folder, and tab "PASTE" at the lower task bar menu to transfer photos into the external storage device.
13.) Selected photo is being copied to the external storage device.