ok thanks, I'll try that

is it risky to do the update I want to try ?

I wanted to try first on the backup NAS to upgrade.
see if everything is ok (except replication, because I need to stop it)

and then upgrade the master NAS (so I have a better idea of the update procedure and how much time it takes).

but I REALLY don't want to loose data, and to be during 6 hours without the NAS working either...