Did you actually build the library or did you just fire up Kodi and expect it to know that you have movies somewhere?

That's normal. You'll have to tell Kodi where your movies are. Once set up it'll find new movies (in this folder) automatically and download infos, poster etc.

How are you exposing the HDD to the Android OS? Can you browse your file structures through something like ES File Explorer or Kodi->Videos?

Normally you connect a hdd and navigate to Video, then Files and its there, no need to add the drive. But I dont think Android TV recognized NTFS drives, exFAT or FAT32 only.

You might want to try a plugin that enables NTFS compatibility.

https://www.androidheadlines.com/2017/07/android-tv-gets-ntfs-usb-plugin-paragon-software.html

https://www.paragon-software.com/us/technologies/exfat-ntfs-fat32-hfs-android/#

Kodi for Rapsberry Pi or Windows doesnt have this problem.

Kodi's library setup can sometimes (often...) be unintuitive for most people. Check out the Kodi wiki for an excellent guide on what needs to be done.

That being said, you can use the Files view to simply play the media, if you don't care about poster art and summaries.