Hi John,
I assume that your "UnionSine Expansion External Hard Drive" is a USB device and that it is empty!
OK - FIRST for the B2 to use this device it MUST be formatted using FAT32. To achieve this, plug it into the B2's USB A port and using the front panel and round knob go through the menus thus.
"main menu > settings > maintenance > Format USB A"
wait for this to complete - blinking red light stops.
Now power off the B2 and move the external hard drive USB plug to the B2's USB C port. Then Power the B" on again.
We are now ready to make a backup of the contents of your B2. To do this, again using the front menu system go
"main menu > USB menu > Export to C"
This will start the backup - now leave the B2 alone to do its thing until it completes (hours/days depending on the size of your music) it is finished when the red light stops flashing.
When this happens please press and gold in the "Stop/Play" button until "Saving" appears on the front display.
Once the clock comes back on the display, please power off the B2 and disconnect your external Hard Drive.
You now have a backup of your music. If you add more music to the B2 you can repeat the "Export" function as the function is incremental.
Hope this helps
Fred