Not your fault, I failed to explain it fully 
- Use Rufus to write the image as per the online doc
- Once the image write is completed, and before you plug USB into SBC, extract the contents of the attachment to the USB pen drive (overwrite existing files/folders).
- Eject, plug into SBC, power on.
- You’ll need to ensure the target USB device is sda and the USB installer is sdb. This depends on the BIOS and ports used (No way to know, that I can think of). Therefore, If the install fails to unmount, rewrite the image using Rufus again and swap the USB drives over on SBC.
- Select Install DietPi sda.