In OSMC, sometimes the SD card is full. To find the largest file, first connect to OSMC with SSH
ssh omsc@192.168.1.15 (your ip address) – password: osmc
sudo du -hs /*Maybe your home folder is too big.
You can narrow it down with:
du -hs * .kodi
and if .kodi seems to be using the space:
du -hs .kodi/*
forest image from jplenio (pixabay)