A:
When you say "accidentally downloaded the new windows 10" did you mean to say you installed Windows 10 on your external hard drive? If so, the data that was on the drive before installation of Windows 10 has been written over. There may still be a chance to use commercial-grade recovery software and tools to recover the data, but it's a long, expensive shot. Moreover, If Windows 10 has been running on the external drive, then your recovery chance decreases since Windows 10 will have written over the data area as many times as you booted up Windows 10.
You can try to plug this drive into another computer with recovery software installed and see if you can read the deleted data (using recovery software).
Good luck!