Hello jackie_w, thank you for your help
I have already done this, with the H2O of my wife who is "pure", and I meet the same problems : As soon as I disconnect my kobo properly after having used Calibre to install my books, the database is updating, but stops after few percents and I have to swich my kobo off...
And then, sometimes the database works, sometimes not.
Fortunately my wife is happy with just few books in her reader, so I don't have to spend a lot of time to reinstall everything... but it is really anoying.