I didn't chip in because other users have suggested options already, I don't see myself as the appropraite first contact point when users run into problems. I strongly recommend local dealer as the first contact point on all after sales support issue.
There isn't a lot of feedback on this topic, I assume most users don't enconunter similar problem, maybe they prefer to copy files to SD card via card reader, maybe they didn't encounter similar problem when they connect their N7 DAP to their PC. I belongs to the later group which might sound discouraging to you.
Since I can't suggestion any solution to resolve your problem, I can only provide a detail record of what I have encountered when when I copy file from my desktop PC to the internal storage and SD card of my N7 DAP.
1. This is before I connect the DAP to my desktop PC
2. When I connect the N7 to my desktop PC, Window will pop up a window immediately. At this point, I didn't insert any SD card into N7, so only Internal Stoarge is displayed in the pop up window.
3. Wipe down the Notification Center of N7 DAP, you can see the N7 DAP has detected the connection and set to File Transfer already
4. For the recrod, you can set the default action of USB connection at Setting > USB > Default USB configuration
5. To illurate the file transfer capability, I copied the Raven by Rebecca Pidgeon (DSF) from my PC to the internal storage of N7 DAP.
6. File transfer completed,, right-mouse click to display the file property of the new folder: 1.8G in N7 internal storage
7. Insert SD card into N7 and repeat the file copying test
8. The pop up windows now displayed both Internal Storage and SD card options.
9. Repeat the file transfer test, the file property windows stated Samsung SD card as final location.
10. The content of the DSF folder
Last but not least, the USB A to C cable is a very old OTG cable bundled with Cayin DAP few years back. Nothing exotic here,