Impossible to copy "Bibliothèque Photos.photoslibrary" with "Synology Drive"

Then you'll have to export the original image files to folders

and copy those folders to the EHD.

By checking the Export IPTC as XMP checkbox you'll be including any metadata, title, keywords, descriptions, etc., that you added to the image in Photos with the image. When the two files are imported into another Photos library together the metadata will be added to the image in the library.

To add to Terence's warning about NAS drives, the Photos library has a complex ecosystem of file permissions and hard links that the file system of NAS drive so not support. You may be able to copy your library to it eventually but the chance of getting a working copy back is minimal.

If you value your library and the work that you've put into it I wouldn't use the Synology to backup the library.

Simply, a NAS is not a suitable environment for a Photos library. This os for two reasons: one is that the disk format is inappropriate and the second is that accessing over a network share is not supported. Therefore, putting your library on a NAS has a high risk of dataloss.

Tks for your answer but a Nas is perfect for backing up files

But you're not backing up files, you're backing up a complex database that tracks multiple versions of your data. A NAS is not a suitable place for a Photos Library. End Of.

Here's your answer: you can't. Why is that difficult to understand?

If you want to manage your photo files yourself then don't use Photos. It does not have the tools for the job. Use another app, there are plenty to choose from.

Thanks but I want to have a copy of my photos on my Synology, it's just my choice ;) It used to work perfectly and since Mojave I think, it doesn't work anymore...

Then downgrade to Mojave, where it worked for you. Keeping the referenced files on a NAS has never been supported, even if it worked for you, and the new security features on Catalina are adding an additional level of complication. And there have been a few new bugs reported for Photos 5.0 on Catalina; referenced photos in an iCloud Photos Library can cause the iCloud sync to hang; referenced photos are currently not shown on the Places map. I would not even think of using a referenced library currently while running Catalina, NAS or not.