I'm not running Plex for a while and instead want a HDD attached to my MiBox, what would be the best app for this? I know I can use Kodi but that seems like a lot of setup, the Plex app was really nice and convienent so something similar would be great.
You might want to keep using Plex... Plex can play local files
The best thing about Plex is the media library and metadata and transcoding. as a pure player it is nothing special, with lots of issues really. You're better off with Kodi.
Kodi will play off an external hard drive. I dont think theres a Plex skin/theme, but theres tons of them out there, one of them is bound to tickle your fancy.
Kodi would do the job, just add your hdd as a source for your library and it will fetch all the meta data.
To make the UI as Plex like as possible install Embuary https://forum.kodi.tv/showthread.php?tid=320137 . It is a skin to mimic emby but still looks pretty similar to Plex.
To get a continue watching feature like Plex, install LazyTV addon from official repo, it will generate a continue watching list that you could point to a home page widget.