Advise on XMP rules

[Roadrunner]

I have now scanned 40.000 photos as high quality scans in 16-bit tiff format. The sizes are between 100 and 500 Mb each.
All photos are located on a Synology NAS and I backup to both external drives and cloud.
As standard PSU uses XMP standard to store XMP. The tiff's will have embedded XMP. The problem for me is when I do my cataloging all changed tiff files will backed up again every time I change metadata and that is a slow process. Are there any disadvantage to change the setting to 'Always as a sidcar''? That will be a hugh difference on backups when cataloging and updating metadata. Any drawbacks before starting doing that? I have tested to open such file in Adobe PS and that looks fine.

[Hert]

For PSU it doesn't matter. But for interoperability with other software it will matter. For TIF files, software should check if there's embedded XMP data inside the file. You should never expect any software to check for a XMP sidecar file for TIF files (though they may, or offer an option such as in PSU).

As long as you are aware of this fact you should be fine.

You mention Photoshop. Try this:
1. Write the XMP as a sidecar file in PSU
2. Open the TIF file in PS (not the XMP sidecar file)
3. Check if there's metadata displayed
4. Close PS
5. Open the XMP sidecar file in a plain text editor
6. Make a change
7. Save the file
8. Start PS and again open the TIF in PS
9. Check if the change is reflected

[Roadrunner]

Thank's Hert!

PS read the sidecar file fine, but PS also embed the metadata in the tif file when opening in Camera Raw which I always do. Even if I didn't save the file in PS.
I can't find any settings to avoid PS to do this.

I have decided to go with 'XMP standard' instead to avoid running into future issues.

[RobiWan]

The problem for me is when I do my cataloging all changed tiff files will backed up again every time I change metadata and that is a slow process.

I don't know what kind of backup software you are using. For tasks like this you "need" software like "Macrium reflect", "Syncovery" or something else, that can backup only changes sectors