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LR normally doesn't modify photos when you view or search them. So I think something else is going on in your situation.
LR will write .xmp sidecars for raws and modify JPEGs, TIFFs, and PNGs when their metadata actually changes. There are some things that can happen in background that will change metadata, e.g. LR adding address suggestions (reverse geo-encoding) or syncing with LR Mobile.
To troubleshoot what's going, grap a copy of a photo before you start LR and then after LR has modified it. Upload the two copies to Dropbox, along with screenshots of the Finder showing the exact modification dates. I can put the files under the microscope and determine the exact changes, why LR might have made them, and likely how to avoid the situation.
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You need to turn off "automatically write changes into XMP" in Catalog Settings->Metadata and turn off "Include Develop Settings in metadata inside JPEG, TIFF, PNG and PSD files" in the same pane. It is not needed to write changes into those files as all changes are stored in your catalog.
Also, Be EXTREMELY careful with having a lightroom catalog on your dropbox folder. It is very easy for it to become corrupted if you forget to close it on one computer and get dropbox to sync it from the other. Really bad idea. You are far better off putting your catalog on a SSD USB3 hard drive (cheap, fast, and they are absolutely tiny) than using dropbox because of this reason. If you do use this, make sure you backup the catalog often to a folder outside of your dropbox folder.
You need to turn off "automatically write changes into XMP" in Catalog Settings->Metadata and turn off "Include Develop Settings in metadata inside JPEG, TIFF, PNG and PSD files" in the same pane.
Even with those options enabled, LR shouldn't be modifying files as the result of browsing and searching the catalog. Something else is going on here.
There are many reasons why it will do that while just browsing through just imported images. If any keywords were set on import. If any default copyright notice gets set on import. If it is doing a reverse gps lookup it will write those settings. If any default develop settings are set. Even if there are no default settings it will still write xmp for the Lightroom default develop settings. Only way to prevent this is to turn it off.
Right, I alluded to those things -- they are among the many ways that a photo's metadata might get changed as a result of a user action.
But once these metadata changes have been written out for each photo, there should be no more file modifications until you change the metadata again through one of those actions. Browsing in the Library is not one of those actions.
LR 6 does a very good job of writing the XMP changes in background, and it will warn you if you try to exit when not all the changes have been written. So if you exit LR, and it doesn't warn you that it's still writing XMP, and then you restart LR again and just browse the library, there should be no modifications made to files during that second session. But that's what Andreas thinks is happening to him.
If in fact that's what's happening, that's a bug and should be reported. But if in fact Andreas is doing some action other than browsing that is legitimately changing the metadata, it would be good for him to understand that.
My guess is that this is a side effect of using the catalog on the dropbox folder. The file's modification dates are probably out-of-sync between the two sites as they get synced at a later time by dropbox then when they get written by Lightroom. On the other computer when you open Lightroom there, the xmp (or normal jpeg/tiff/etc) appear to be out-of-date as they don't have the correct modification date and time and get marked for rewriting the xmp. If these are just xmp files that's not a big deal but if they are files where you can write the xmp inside the files this creates a hug bandwidth issue that just keeps repeating itself as when you now go back to the other machine where it has now synced all these files, they turn out to all have incorrect modification dates again! I don't think this can be avoided when you do the dropbox sync thing as it is fundamental to Lightroom's behavior and how dropbox operates I think when you have the automatic xmp writes turned on. Only solution is to turn the automatic writes off.
You guys are reinforcing my decision to stay away from writing to .xmp and relying on Lightroom's default procedure which is to store the info in the Catalog File.
I have the Adobe CC package so I have the use of Lightroom and Photoshop and use Lightroom exclusively to work with my RAW files, and I have no interest whatever in using Bridge or ACR. The last version of ACR that I used was 2.4 and that was prior to the release of Lightroom.
Lightroom allows me to adopt a workflow that excludes the use of ACR and .xmp sidecars.
I have no problem accepting that there are some users that prefer to work with their raw files in ACR but I believe that using both at the same time is causing more problems than benefits.
Working with Lightroom the info is in the Catalog.
Working with ACR the info is in the file.
"The file's modification dates are probably out-of-sync between the two sites as they get synced at a later time by dropbox then when they get written by Lightroom."
I don't think this accounts for the symptoms. Even if Andreas started LR before all the photos and sidecars were synced, that wouldn't cause LR to write them. The only thing that triggers LR to write out metadata is either an explicit Metadata > Save Metadata To File command, or if Automatically Write is enabled, a change within LR to a metadata field. Changes by external applications to a photo/sidecar can trigger a metadata "conflict detected" status, but it won't trigger LR to write out the metadata.
You can test this out for yourself:
1. Enable Automatically Write Changes.
2. Select a raw file and change its caption; wait until the .xmp is updated on disk.
3. Save a copy of the .xmp using the Finder or "cp -p" on the command line.
4. Change the caption again; wait until the .xmp is updated on disk.
5. Replace the .xmp with the saved copy from step 3.
At this point, there is an "old" .xmp in place that doesn't match what LR last wrote from the catalog. LR won't overwrite that .xmp until there is another change to the photo's metadata within LR.
So something else is going on.
Is this behavior still occurring? There have been two other reports of similar behavior, neither involving syncing with Dropbox: Metadata (and so whole) PSD is updated when you export the file from your Library in Lightroom, why?? | Photoshop Family…
It would be good to get more details and try to narrow this down to provide an actionable bug report to Adobe.
Thank you all for really good, in-depth answers! I've now removed "include Develop Settings in metadata" and also the "Adress Lookup"-function. I also switched off the "write data to XMP". After re-launching Lightroom, it seems to have done the trick. I can now browse my cataloge, and only the database-files (previews etc) get's updated. Really good.
I later tried to turn the "write to XMP" back on, to see what effect that might have. As you have already pointed out, it had no effect towards what I was experiencing - a complete re-write of the files every time I viewed them.
Attached is a screenshot of how Dropbox now behaves when I simply browse.
Now, I cannot confidently answer all the points you both made through-out the list of replies, johnrellis and Jao Vdl but I can perhaps just briefly say that I took your warning about using Dropbox in this way seriously. I now keep updates of the catalouge on different places as well, and also on a TimeMachine spare. I'm also very strict with closing Lightroom at work (waiting a few minutes) before opening it at home. I was before as well, but now even more so.
Again, thanks so much for your help, really appreciated. I hope the odd behaviour pops up again, and if it does at least now I know I should report it as a bug.
I do not write info from Lightroom to xmp but rely on the info in the Lightroom Catalog file, which is the Default procedure for Lightroom.
Just one hint in Lightroom Preferences> General Tab> Prompts, click on "Reset all warning dialogs", if you turned this off then you may not be seeing the warning when the info in the Catalog is not in sync with the info in the file i.e XMP. there should be an option there to update the data to the file info or retain the catalog data.
The danger here if you update the Catalog file to reflect info and edits done in other applications i.e ACR then work done in Lightroom may be replaced/lost.