SOLVED !
Have you formatted the HDD in the box?
Well, I just finished initialized it from the menu Parameters > System > Expert > Hard Drive > Initialize. And ... it worked !
At first, I tried the same process but from the image openpli-8.0-release-dm8000-20210111. As that didn't work, I formatted it
from my desktop (Kubuntu 20.10) with the KDE Partition Manager, inserted it in my DM8000 (re-flashed with
openpli-7.3-release-dm8000-20210119 because I read this comment), and tried a Verify Filesystem but I got an error at 99% (on mounting the device).
I tried again from a shell with the problem listed above. I didn't think re-format it again .
Apart from the UUID, the parameters shown from the different commands (tunefs, parted, sfdisk) above are exactly the same.
Thanks all for your help !