Disclaimer - I have not done what you're doing (yet) but I do know linux quite well.
My guess is you're accessing your windows files through an icon that was created on your desktop when you first logged in. The OS will detect mountable media (disk volumes, USB drives, etc) and present them as being available in this way. This is similar to OSX. The problem with this is that it doesn't always make it obvious how to access this from programs like PI.
If I'm using Linux to access other OS partitions regularly via scripts and software like PI, I prefer to create a fixed mount point. The first thing you need to do is determine which partition your windows data is on. From there you simply edit the file /etc/fstab and add that partition to a mount point.
Some helpful information is here:
https://ubuntuforums.org/showthread.php?t=1880423If you really get stuck PM me - if you can get a screen share going I can probably help you out.