Thanks Larry
Thats pretty much how I see it although I can't see any sidecar files, what extension are they and where would they be?.
My files are jpegs from a Canon EOS7D.
Craig
Separate sidecar files are not necessary with jpgs because metadata can be written directly to the file itself. This is also the case with a few other file types such as (I believe) tiff and psd. When separate sidecar files are needed, and when Lightroom is set to save metadata to the "file" rather than only to the "catalog," it creates them in the same location as the original file, and it uses .xmp for the file type. I don't know if the xmp file type is a "standard" sidecar file type for other programs, or if this is just what Lr uses.
The Lr error may be a somewhat generic error message that is pointing out a problem with the metadata structure -- i.e. it may be using the word "sidecar" even though jpgs don't use a separate file for this data.
Either way, I've heard of this sort of error before when using more than one program to edit photos -- i.e. one program will alter a jpg in such a way that another can no longer write to it. I always recommend keeping an untouched, original version of the photos saved in order to deal with situations like this (just in case the the problem cannot be undone.)
The next step would be to determine if the problem is limited to certain data fields (i.e. certain tags). Does writing ANY data to the file in MC cause this issue, or is it just certain fields? Is it perhaps just custom fields -- or maybe even a specific custom field -- that causes problems? Try to narrow down the issue as much as you can, and see if it points to something that others could check.
One other question: Is MC still running when Lr has the problem? Is there any way to test the same files on another system running Lr? One thing you could try would be to delete a "problem" jpg from the Lr library (but NOT from the drive) and seeing if it still doesn't work with Lr. Just remember to use COPIES of the files for your tests so you don't lose any originals.
Larry