@threeOh @GBarrington Thank you both for providing your solutions!
Here is how I solved it in case anyone would have a similar issue:
First, my Lightroom Classic version is 10.1.1; Lightroom version is 4.1.
The fundamental issue is that Light Classic "sync function" does not truly upload the photos to the cloud.
It only uploads a "smart preview" (a few hundreds KB heavily compressed dng file) - not the original raw or jpg file.
Also, attention that if we delete a photo from Lightroom Classic catalog (when sync is enabled), it will be deleted from the cloud as well.
Now let's consider a real case:
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Computer A: Lightroom Classic (synced)
Catalog:
All Photographs: 500
All Synced Photographs: 500
Folders:
Main: 400 (local directory)
Cloud: 100 (sync directory)
Computer B: Lightroom
(Actually it also applies when there is no Computer B, both Lightroom Classic and Lightroom are installed on Computer A.)
Both Computer A and B can see 500 photos. But these 500 photos are in different situations:
On Computer A - Lightroom Classic,
- 400 raw files which were previously imported into Lightroom Classic, now sit on local disk.
- 100 raw files which were downloaded from the cloud (these were not in the same directory as those 400).
On Computer B - Lightroom,
- 400 smart previews, all edits are synced, they are NOT the original photos, but heavily compressed (noticeably less sharp)
- 100 raw files which were previously imported into Lightroom directly (so end up in the cloud). These are what I consider truly "in the cloud".
******************************************************************************************************************************
After several hours of research, I realized that it is not a good idea to use Lightroom Classic and Lightroom at the same time.
Either choose Lightroom Classic for its powerful features or Lightroom for its flexibility to work across different devices!
Make up the mind to select one of them!
Then next is to solve the inconsistency (methods for both selections are provided):
**Please be very careful about the following steps. Some operations cannot be undone!
**Backup of EVERYTHING (both local and cloud) is strongly encouraged beforehand!
1. *Recommended* Stop using Lightroom (the logic of the cloud is VERY confusing)!
How to migrate the 100 photos in the cloud to the local catalog on Computer A:
1) Uninstall Lightroom on Computer B (in case of any mistakes).
2) On Computer A Lightroom Classic, Preference - Lightroom Sync - Location, click "Specify location for Lightroom's Synced images" and "Use subfolders formatted by capture date". Let's say the location is /XX/Cloud.
3) Enable sync. When sync is done, turn off the sync! The "Cloud" folder should show up in the left "Folders" panel.
4) In the left "Folders" panel, manually move all sub folders of "Cloud" to the main directory.
5) Never turn on sync again. Otherwise there will be duplicates added to the catalog (unless the cloud is empty). Uninstall Lightroom on Computer B.
2. Stop using Lightroom Classic. Upload those 400 raw files to the cloud:
1) Turn off the sync of Lightroom Classic on Computer A.
2) On Computer B Lightroom, use filter "Synced from Lightroom Classic", remove all 400 results (don't worry, these are just smart previews).
3) On Computer A Lightroom Classic, export those 400 raw files as a catalog.
4) Copy the catalog to Computer B. In the Lightroom interface, use "File - Migrate From - Lightroom Classic Catalog" to import all raw photos. Then it will automatically upload the 400 new photos to the cloud. (If there is no Computer B (both Lightroom Classic and Lightroom are in Computer A), and it pops an error "The Lightroom Classic catalog appears to be an exported catalog or has been copied", just double click the catalog to open in Lightroom Classic and close it. Problem magically solved! Thanks to
https://www.youtube.com/watch? v=RK9PCD1Sj5A.)
5) Now the cloud has 500 raw files in it.
6) Uninstall Lightroom Classic on Computer A.