Fire OS 5, the latest version of the Fire TV operating system, ships on the 2nd-gen Fire TV and is coming to the 1st-gen Fire TV and Fire TV Stick soon. As you probably know, Fire OS 5 is based on Android 5.1 Lollipop. With the upgrade comes Androids new tighter restrictions on app permissions. One of those new restrictions in Android forces apps to explicitly ask for your permission before they’re allowed to modify files on external storage devices. Some apps, including Kodi, have not yet been updated to request these new permissions, so are unable to modify files on a microSD card or USB drive connected to a Fire TV running Fire OS 5. This is why you cannot currently fully move Kodi to external storage on Fire OS 5.
Kodi, and all other apps, have no problem reading files from external storage. This means you can play media files, stored on a USB drive or a microSD card, with Kodi without any issues on the 2nd-gen Fire TV. Where you run into issues is if you change Kodi’s advanced settings to use external storage for other purposes, like storing your database or thumbnails. Read and write access on internal storage for all apps is unrestricted in Fire OS 5 and Android Lollipop.
Some apps, like ES File Explorer, are already updated to properly ask for permision and can have full write access to external storage. Kodi’s developers need to implement Android Lollipop’s new external storage access API before some of its advanced functionality will work with Fire OS 5 and any other Lollipop based device. Once that’s done, I’ll update my guide for partially or fully moving Kodi to external storage. These stricter permissions in Android do not affect moving apps to external storage within the Fire TV’s application management settings.