It's a common occurrence to encounter the "low on storage" popup on devices
with meagre storage, such as the Fire TV Stick, when running Kodi. Almost always
the cause is the amount of cached images in the folder userdata/Thumbnails.
Kodi thumbnails cache can grow over time and depending on your playing habits and media
library collection can reach several gigabytes. Also Kodi does not automatically delete
unused thumbnails. If you delete a video its cached artwork remains in the Thumbnails folder
Luckily, Kodi will re-cache needed artwork when required. So the easiest way to fix the
low storage warning is to delete the userdata/Thumbnails folder and its accompanying database
Database/TexturesXX.db. This function does just that.
If you're storing your media on a NAS or a large external drive, it's a good practice
to export your library from time to time. In Settings:
This will store a video's artwork and nfo file on the drive with the video. When Kodi
needs to re-cache it will be almost instantaneous. This also makes rebuilding your library
quite easy should some disaster occur.