The Lightroom Previews folder (& sub-folders) may look like they contain image files, but not so.
You must consider the Previews folder to be a 'secret' cache folder that is solely intended for Lr to read and write (and delete) the previews as it works (in real time).
It only works if it remains beside the Catalog file (.LRCAT) that controls the previews.
Now the LR CAT is not recognizing the previews file.
You can delete the entire Previews folder ( your catalog name PREVIEWS.LRDATA) and when Lr runs it will re-create the previews as it needs them ( perhaps slowly, as you look at more photos and folders in the Library). There is no "re-sync" required, although you can give the Menu command to re-build all previews.
Uploading or backing-up the Previews folders is time consuming (55Gb) and basically worthless.
Your images that synced with Lr Mobile are those that are in Collections , marked with the sync icon. The Previews cache does not influence this.
The Catalog file ( .LRCAT) is your prime, and most important, database, and must NOT be deleted.
And you must have your original image files available for Lr to reference to be able to develop, export, print, and 'export' to your mobile device.
Thanks for your feedback.
My laptop was in repair at mac and just got back on it.
Now I am trying to rebuild the previews file as you suggested.
"although you can give the Menu command to re-build all previews."
I only see build smart previews in the menu - Library - Previews - Build smart previews.
I had to reconnect my raw files, and they are building, but it seems to be adding to the original file sixe of 55GB.
If this is the case it will be double that when it finishes 110 GB.
Way to big.
How do I "rebuild", without having to trash the original previews file?
I only see build smart previews in the menu
Strange! this is what I see- (Windows-PC) "Build Standard-Sized Previews"
There is no harm in entirely deleting a Previews folder (xxxxx Previews.LRDATA) and 'deleting' is often recommended to fix problems.
Lightroom will automatically re-create the previews any time you open Lightroom. (No menu selection necessary!)
The size of the previews folder depends on several things, primarily the settings in the "Catalog Setting" Preferences, and the amount of Develop work you do.
There is no need to make "Smart Previews" unless you have a special reason, such as- removing a drive that has the original photo files. Smart Previews will create a folder specifically for these, separate from the regular Previews..
In your first post you say - "LRCAT is not recognizing the previews file.."
This is a good reason to delete ("trash") it (the .LRDATA folder) from your computer entirely, and let Lightroom automatically re-build it.
Nothing about the "Previews" folder concerns the Mobile sync- if problematic, that would be a separate problem.