The internet-box is fully compatible with an "old" version of ext4 (HDD formatted directly in ext4 format with ubuntu 14.04 kernel 3.13.nn). If I have time, I reinstall an old VM with Ubuntu 14.04 to try it again.
But it doesn't explain the difference between a newly formatted HDD in ext4 and the same converted from ext3 to ext4 (ubuntu 18.04 kernel 4.15.nn (any kernel linux/ubuntu freaks there ?)).
Seems that an improvement in ext4 (kernel 3.13 --> 4.15) formatting procedure make it no more compatible with internet-box ?
PS : read TB instead of GB in my first post... tired...