In previous versions of HDR Efex, the merge would fail if the intermediate merged.tiff file exceeded 4GB (the limit for “standard” TIFF files). This would happen even if the final HDR output (compressed) would be well under 4GB. This makes it difficult if not impossible to consistently work with images larger than about 300 megapixels in the program.
Implementing BigTIFF or PSB support for the intermediate file should avoid this issue.