Unfortunately, Lightroom flags ('Rejected'/'Flagged') are internal to Lr catalog: these flags are not imported from XMP sidecar files and not exported to XMP.
You may use standard XMP Label or Rating, than convert it to Rejected after culling:
1) Export XMP Sidecar files for your catalog (or selected files) from Lightroom: select files (all, or by-folder) and use Menu - Metadata - Save metadata to files to export XMP sidecar files
2) Cull with FastRawViewer, assign, for example one star for rejects and 2 star for keepers (or use some color label for rejects)
3) In Lightroom: select all files again and use Menu - Metadata - Read metadata from files
This will update ratings/labels from XMP files (i.e. set by FastRawViewer)
4) Create smart collection with rule 'rating is equal to 1 star' (or, if you've used label, create smart collection by this label)
5) Select all files from collection and set flag to Rejected.
Unfortunately, Lightroom
Unfortunately, Lightroom flags ('Rejected'/'Flagged') are internal to Lr catalog: these flags are not imported from XMP sidecar files and not exported to XMP.
You may use standard XMP Label or Rating, than convert it to Rejected after culling:
1) Export XMP Sidecar files for your catalog (or selected files) from Lightroom: select files (all, or by-folder) and use Menu - Metadata - Save metadata to files to export XMP sidecar files
2) Cull with FastRawViewer, assign, for example one star for rejects and 2 star for keepers (or use some color label for rejects)
3) In Lightroom: select all files again and use Menu - Metadata - Read metadata from files
This will update ratings/labels from XMP files (i.e. set by FastRawViewer)
4) Create smart collection with rule 'rating is equal to 1 star' (or, if you've used label, create smart collection by this label)
5) Select all files from collection and set flag to Rejected.
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Alex Tutubalin/FastRawViewer team