[Milhouse]

Wouldn't that be slower than using the internal storage? Especially when scrolling and it needs to load all those thumbnails.

It might be a tiny bit slower, but with a good SD card/USB memory stick I doubt you'd really notice (by "good" I mean an SD card with good 4K random read/write performance - look in the Raspberry Pi threads, this gets discussed *a lot* - Samsung EVO SD cards are usually recommended).

[Namoi]

I just pump here but i had s similar issue absolutely not related with kodi on my phone.
If you get errors on your card (internsl or external) android media manager will start sucking a lot of space (resolved by rebooting but just for a while).
An easy fix is to plug the device on a windows system and run check disk on it.
Maybe irrelevant to this problem, so sorry in advance, but worth a try though.