Network Import just partially transfer

I have an old My Cloud (on OS 3) and I bought a new My Cloud Home.
Via the Web app I started the Network Import from the old to the new NAS.
Both NAS are on the same local network, it should be an easy task then.
I started the process overnight, the day after the service seems to have finished, but only a partial copy has been done.
How can I check what’s wrong ?
Maybe activating some logs ?

Thanks