ReadyNAS 31400 crashed during Firmware Upgrade from 6.7.5 to 6.8.0

I have a RN314 with 3x4TB + 1x6TB HDD in one single RAID 5. Until this afternoon everything was fine and I tried to upgrade to fw 6.8.0. In the same way as usual. During the upgrade a message came up like: "Unable to write the files". I pressed "retry" and the box continued with the Reboot. After the reboot the display showed continousely "Failed to Startup"/"Retry Starup" and the Blue On/Off-button was flashing all the time. After about 1.5 hours the display shows now the hostname and ip - as usual. Also the four disk lights are on and green. But still cant connect to the Administrative interface. Also no shares available. But I can connect via SSH.

What I unfortunately realized later on is that the usage of the system-volume root is min. 90%. So I did a df -h which shows:

Filesystem      Size  Used Avail Use% Mounted on
udev             10M  4.0K   10M   1% /dev
/dev/md0        4.0G  3.6G   49M  99% /
tmpfs           2.0G     0  2.0G   0% /dev/shm
tmpfs           2.0G   29M  1.9G   2% /run
tmpfs           984M  204K  983M   1% /run/lock
tmpfs           2.0G     0  2.0G   0% /sys/fs/cgroup

Dont know whether this is relevant in this context.... I also wouldnt absolutely dare to to a "reboot" via ssh. Any ideas what I else could do? (At the moment I do have about 7TB of data on the system, out of ~ 10.9TB).

kind regards,

Christian