Voici la réponse du support Syno 48 heures après ma demande:
Thank you for the inquiry.
Please notice DiskStation will make the re-indexing for the multimedia files stored in the folders "video", "music" and "photo". Unless the re-indexing is done, DS won't enter the hibernation mode. Please login DS WebUI and check if two processes "convert" and "synoindexd" are occupying the most CPU resource. This means DiskStation is working on re-indexing for the multimedia services.
Also note that when these services are enabled, the internal HDD cannot enter hibernation mode:
* Weblizer
* Local Master Browser
* eMule
* Download Station
* Squeeze Center
* Mail Station
* Surveillance Station
* Log is enabled in [Media Server]>[Diagnose]
* DiskStation will re-indexing the multimedia files after firmware upgrade. There will be index/convert process under resource monitor when this occurs
Hope this helps.