Known issue of FireOS 3. For a true fix Research updating from PreRooted FireOS 3 to PreRooted FireOS 5. For a short temp fix try: https://forum.xda-developers.com/fire-tv/help/hdd-space-shrinking-fast-firetv-box-t3518141I had a new firetv1 at version 126.96.36.199 and I got it rooted using towel root then followed OP's instructions. When the last command was run it rebooted however now it get stuck on the opening screen saying the the firetv is completely out of space and when I click on manage application the box to manage applications is empty. I pushed a rom to the SDcard folder in adb before I did the bootloader unlock but I don't know if that's the issue, it was only about 400 megs I also tried a factory reset but the out of space dialog still pops up with. I can use a keyboard to press escape to and it will let me go through the setup but after the opening video it just goes back to the beginning again. Anyone have any insight into why the out of space dialog box is showing up and how can I fix this? I ordered a cable to see if I can get into fastboot and if its truly unlocked but that will come in a couple days. Any help is appreciated thanks.
That temp fixed messed up my installed apps. They would not update or reinstall since the dbcache had been deleted. It's just a temp until your ready to upgrade to FireOS 5.