The rootfs is your system partition. Check the space “df -h”. If you’re able to install pyload, I’m sure you can uninstall other 3rd party apps you’ve installed. “dpkg -l|grep packagename” to find out the name, “dpkg --purge packagename” to remove including its settings. Do this with caution or you might ended with a brick.
WDMyCloud:/var/log.hdd# apt-get clear
E: Invalid operation clear
WDMyCloud:~# du -sh /
du: cannot access `/proc/10764/task/10764/fd/4': No such file or directory
du: cannot access `/proc/10764/task/10764/fdinfo/4': No such file or directory
du: cannot access `/proc/10764/fd/4': No such file or directory
du: cannot access `/proc/10764/fdinfo/4': No such file or directory
538G /
WDMyCloud:/var/log.hdd# dpkg --purge pyLoadCore
dpkg: error: failed to write status database record about 'vim-tiny' to '/var/lib/dpkg/status': No space left on device