j'ai posé la question directement chez Synology, voici leur réponse
"Greetings Francois,
I'm sorry to inform you that Shared Folder Sync is not supported for DS209j due to hardware limitation. If this is as an important factor of yours concerning with backup occasion, please consider upgrading the DS209j to other newer models.
For your reference, the models support Shared Folder Sync are listed as below:
5 bays: DS1511+, DS1010+, DS509+, DS508
4 bays: DS411+, RS810+, RS810RP+, RS409+, RS409RP+, DS409+, RS411, DS411j, DS411slim, DS410, DS410j, RS409, DS409, DS409slim, RS408, RS408-RP, DS408
2 Bays: DS710+, DS211+, DS210+, DS209+II, DS209+, DS211, DS211j, DS210j, DS209
1Bays: DS110+, DS109+, DS111, DS110j, DS109
To achieve backup between Synology products, apart from using Shared Folder Sync, is also eligible for using our Network Backup feature.
Your understanding will be well appreciated.
BR,
Antoine Yang"
tant pis