how to access a file that is located on a different hard drive from within a python script

And another related question: how to access a file via network from within a python script; the Network is a samba share.

I tried different things, but can't figure it out. I keep getting: No such file or directory

I am running the script like this:

$ /usr/bin/python3 "/home/nathaniel/Documents/my first python script.py"

This is my mount:

/media/nathaniel/ssd/

This is my file:

/media/nathaniel/ssd/test/SD/1.avi

I have been trying different things, but nothing works.

This is a simplified version of my script:

#!/usr/bin/env python
import subprocess
import tempfile
sourcedir = "/media/nathaniel/ssd/test/SD"
diroutput = "/media/nathaniel/ssd/test/HD"
sourceExt = "avi"
def getFileNames():
    global filenamefirst 
    filenamefirst = int(input("enter file name NUMBER to start with: "))
    global filenamelast 
filenamelast = int(input("enter NUMBER of last file to process: "))
def main():
    import os
    import os.path
    getFileNames()
    for x in range(filenamefirst, filenamelast+1):
        if os.path.exists(diroutput + "/" + str(x) + ".mp4"):
            os.remove(diroutput + "/" + str(x) + ".mp4")
        from subprocess import check_output
        check_output("ffmpeg -i \"" + sourcedir + "/" + str(x) + "." + sourceExt + "\" -c:v libx264 -b:v 18M -c:a aac \"" + diroutput + "/" + str(x) + ".mp4\"")
        print("^======= finished file " + str(x) + "." + sourceExt)
    print("end")

main()

the error:

FileNotFoundError: [Errno 2] No such file or directory: 'ffmpeg -i "/media/nathaniel/ssd/test/SD/1.avi" -c:v libx264 -b:v 50M -c:a aac "/media/nathaniel/ssd/test/HD/1.mp4"': 'ffmpeg -i "/media/nathaniel/ssd/test/SD/1.avi" -c:v libx264 -b:v 50M -c:a aac "/media/nathaniel/ssd/test/HD/1.mp4"'