exactly same issue with me today !!!!
Old topic, but I found a solution:
With Lightroom closed, go to C:\Users\"yourusername"\AppData\Local\Adobe\Lightroom\Caches\Sync Data and delete the folder "Sync.lrdata"
Open Lightroom, resume sync, and everything should work.
I got this today too, after update to CC 2015.10 (though there was some separate issue with the Finder (on Mac) that may have caused some hiccup). I don't see a Sync.lrdata file anywhere in the filesystem. Looking at all .lrdata files and all files with Sync in the name have not yielded results.
For Mac, try Users / [your username] / Library / Application Support / Adobe / Lightroom / Caches / Sync Data
I just googled the equivalent of the AppData Windows folder on Mac, and it seems like it's in Library/Application Support. Poke around in there and see if you can find it.
I took another look but still did not find it.
For those faced with this situation (Lr identifying your catalog as a duplicate or backup), first, be reassured that you have not lost any photos/videos. Second, you can patch it up but the list of synced items will be reset. You can re-sync Collections but auto-uploaded items will not show in All Synced Items or Lr Mobile anymore. Here's how:
- In Lr, note which collections are set to sync with CC.
- In Preferences -> Lightroom Mobile, select Delete All Data. Click through the dialog boxes. Your browser will open. Follow the instructions there to delete all items from CC (lightroom.adobe.com).
- Restart Lr (no need to restart your computer). After a few minutes, Lr's All Synced Photographs, and Lr Mobile, will appear empty. This is the "start fresh" that the dialog box warned you about.
- Select Collections that you want to sync from Lr to Lr Mobile.
- Any photos/videos that synced from LrM to CC between the time the Lr Catalog was "corrupted" (identified as duplicate or backup even though the catalog integrity check passes) and now need to be synced manually. To do so, compare your mobile device's Camera Roll to your Lr Catalog. If any items are missing, add them to LrM. (NOTE: Do not add your entire Camera Roll to LrM because that will sync previously-synced photos/videos to CC and then on to your Lr Catalog. If the item was already in Lr, a virtual copy and stack will be created. I'm not sure, but suspect that you moved the originally-imported item to a different folder in your file system (e.g., to external hard drive where the default import location was the internal drive), you may end up with two copies of the photo.)
Bless you, whoever you are. This was driving me crazy.
Thank you so much!
In another forum, Victoria posted a different path to the sync data. I got the information too late, but I imagine it works with less effort and better effect than what I described. -John
If you wanted to try that on Mac, it's Mac HD/[your username]/Library/Caches/Adobe/Lightroom/Sync Data/Sync.lrdata.
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