JohnnyMnemonic

Thanks for the quick response, @aprayoga!
 
I gave it a shot and unfortunately it doesn't seem to work for me.
 
I've upgraded to Armbian 20.11, which includes LK 5.9:
 
XXXXX@helios64:~ $ uname -r 5.9.10-rockchip64 XXXXX@helios64:~ $ cat /etc/os-release PRETTY_NAME="Armbian 20.11 Buster" NAME="Debian GNU/Linux" VERSION_ID="10" VERSION="10 (buster)" VERSION_CODENAME=buster ID=debian HOME_URL="https://www.debian.org/" SUPPORT_URL="https://www.debian.org/support" BUG_REPORT_URL="https://bugs.debian.org/"  
I've created that file and applied the user overlay as suggested:
 
XXXXX@helios64:/etc $ cat dwc3-0-device.dts /dts-v1/; /plugin/; / { compatible = "rockchip,rk3399"; fragment@0 { target = <&usbdrd_dwc3_0>; __overlay__ { dr_mode = "peripheral"; }; }; };  
XXXXX@helios64:/etc $ sudo armbian-add-overlay dwc3-0-device.dts Compiling the overlay Copying the compiled overlay file to /boot/overlay-user/ Overlay dwc3-0-device was already added to /boot/armbianEnv.txt, skipping Reboot is required to apply the changes XXXXX@helios64:/etc $ sudo reboot  
And then after the reboot:
 
XXXXX@helios64:~ $ sudo modprobe g_mass_storage file=/dev/sda,/dev/sdb,/dev/sdd iSerialNumber=1234567890 iManufacturer="Kobol Innovations" iProduct=Helios64  
I see the following in /var/log/syslog:
 
Nov 27 15:26:36 localhost kernel: [ 3714.194647] Mass Storage Function, version: 2009/09/11 Nov 27 15:26:36 localhost kernel: [ 3714.194664] LUN: removable file: (no medium) Nov 27 15:26:36 localhost kernel: [ 3714.194866] LUN: file: /dev/sda Nov 27 15:26:36 localhost kernel: [ 3714.194973] LUN: file: /dev/sdb Nov 27 15:26:36 localhost kernel: [ 3714.195075] LUN: file: /dev/sdd Nov 27 15:26:36 localhost kernel: [ 3714.195083] Number of LUNs=3 Nov 27 15:26:36 localhost kernel: [ 3714.195344] g_mass_storage gadget: Mass Storage Gadget, version: 2009/09/11 Nov 27 15:26:36 localhost kernel: [ 3714.195354] g_mass_storage gadget: g_mass_storage ready  
I've tried with both a Windows PC and a Linux (Debian) PC, with no luck.
 
I can try and provide anything else needed to troubleshoot, though it may take some time as I'm limited to just one post per day.
 
Thanks!