Also keep in mind that a video file can look like the same format, eg mkv... but have very different parameters... eg 264 files might work but not 265.
Play a whole bunch and see which ones work and which don't, then use something like:
https://mediaarea.net/en/MediaInfo
You install that on your pc, and it ads a right click extension to your file explorer (may be hidden by default, click to show more etc).
Then you just click on a video file and run that, see what the differences are between the ones that do and don't work.
Also ask on a more LG smart TV specific forum as they might have the answers you really want without less work.
The No NTFS can be a thing... but also, somethings will only work for drives up to a certain size.
If you have a USB 3 (blue port), try using that instead of the USB 2 one.
I've found the default 'media player' can be useless on my bravia (just an example)... it'll play some things but, others are glitchy or don't work at all.
So you need to use an app like VLC or JustPlayer on your Smart TV. Sideload them if you have to/can. LG sucks for that cause it's not android. Get android for your next Smart TV!
It can also help to force close, clear the cache of the app that's not letting it play, then reopen and see if that helps. And ideally, if you expect problems with certain files, reboot the tv before you even try it (not power off, reboot).
JustPlayer plays DV btw, good to know (videos play green and purple otherwise).
BTW, don't forget to check the usb cables, and maybe reformat the drives you're using.
Also, uninstall any apps you really don't need as most tvs have no space left and they run badly as a result.
Check how big the internal storage is on the smart tv... you might be shocked at how lame it is.
Good luck.