erwinbeckers,
I recommend that you backup the catalog from Lr not with your OS.
In Lr go >Edit >Catalog Settings >General tab. In the bottom window select <Every time Lr exits>.
Next time you close Lr the backup dialog will come up. Select a location for your backup - you need to know where the backup is located in case you need it.
As part of the backup process Lr will also optimize the catalog.
When Lr does a catalog backup it does not overwrite an existing backup file but writes a new backup file each time. So each backup file is complete in itself as per the time of the backup.
That means that backup files will accumulate in your backup location. So from time to time you have to manually delete older backups. I go in there once a month and delete all backups that are older than a month.
You can backup the previews but you would have to do that with your OS or manually since there is no feature in Lr to backup the previews.
By default Lr will discard the 1:1 previews after 30 days, so from that perspective there is no need for a backup of the previews.
You can select a shorter time for discarding the previews or you can select <Never> in >catalog Settings >File Handling tab.
Anyway, as Ian Lyons has pointed out, it might be preferable to let Lr discard the previews from time to time so that it will build new previews when needed.
From that perspective a backup of the previews is also not needed.
