my problem has been solved by myself finding solution here
http://www.vuplus-community.net/board/threads/xbmc-kodi.24002/
in other words for all users who want use Kodi is mandatory:
a) connect to the box a USB Flash Memory (at least 4 gb)
before turning on the plugin Kodi (this to prevent kodi from writing its files in /home/root/.kodi (this is the reason why internal flash memory goes up to the top)
b) the usb flash memory must be mapped as /media.hdd
in the case like me you have unfortunally connected usb flash after having turned on kodi , you will have (as stated in link upper) to log on to the box via ftp and erase folder .kodi in directory /home/root/.kodi
I did so and now internal flash memory is running at 83% making possible to install other plugins
if you think i've been helpful to you I wait for your likes
p.s next goals to me is to understand why using kodi some video got opened with kodi mediaplayer and others with blackhole mediaplayer and why some lacks of audio playback
i'll let you know in proper thread
cheers my friends