Hi Dan, I think I need a little more advice - what is the role of the script scripts/extract_dataset_part.py and the JSON files created when splitting FASTQ files in lib/galaxy/datatypes/sequence.py, and then used by the class' process_split_file method? Why is there no JSON file created by the base data class in lib/galaxy/datatypes/data.py and no method process_split_file? Is the JSON thing part of a partial and unfinished rewrite of the splitter code? On the assumption that not all splitters bother with the JSON, I am trying a little hack to scripts/extract_dataset_part.py to abort silently if there is no JSON file: https://bitbucket.org/peterjc/galaxy-central/changeset/ebe94a2c25c3 This seems to be working with my current attempt at a FASTA splitter (not checked in yes, only partly implemented and tested). Peter