Flatpak doesn't recognize secondary internal Hard Drive

Hi every one, I hope that this is the right section where to post my issue. I’m a new user to Linux (I’m using Linux Mint 21.3), and I have transitioned recently from Windows 10. I installed, a month ago, the Flatpak version from the software manager of my Operative System, however I’m trying to solve an issue since then. On Windows, I used to save my files on a secondary drive, that I used just for storing documents, but when I want to save a document in it (when I click “save as…”), the other drive doesn’t show up. So, I tried to change the permissions (via Terminal and Flatseal) and it didn’t work. I’ve tried in many ways:

/mnt/VolumeD/University
/media/mnt/VolumeD/University

or just:

/mnt/VolumeD
/mnt
/media

And last but not the least: flatpak override --filesystem=/mnt/VolumeD/University --> permission denied(even if in Flatseal --filesystem=host is enabled)
Could anyone help me ? I appreciate your attention, and I also hope in a plausible solution.
P.S. I have to clarify two last things:

  1. The disk has been formatted from NTFS (Windows filesystem), to EXT4 (Linux filesystem) but I kept the name “Volume D”, because I’m used to it since my previous Windows 10 (so the HDD is perfectly integrated with Linux Mint 21.3)
  2. The second one is actually the main reason why I wrote this issue, because the secondary drive actually is recognized both by the system and from the .deb apps (for example Libreoffice, that comes preinstalled), and just some flatpaks don’t recognize it.

Hello @Iulius0107

Try installing Desktop Editors with packages too instead of flatpak:

Or get .deb package right away here: