HDD Recommendation

I use a Pi3 when I'm away from home to run a few movies from Retropie/Kodi and everything has always worked fine but recently, at home with my Mac Mini, WD Ultra 3TB HDD, and iTunes, things stopped working so well. (The WD keeps showing its empty when I go to play a movie, unmounting the disk and then plugging it back in fixes it but that's another story.) So I'm attempting to use one of my Pi3 to run LibreELEC and was just wondering any recommendations you all have for an HDD that will be used to play movies daily. (I have kids) Externally powered or not? Also, I have about 1000 movies currently. Is there a max/optimum amount of movies Kodi/LibreELEC can handle? Any help would be appreciated!