What is PSNormalizer / normalizerd process
I believe I now know what was causing this. I attempted to print to a Kyocera photocopier that doubles as a fax machine. This process seems to have been going around and around, attempting and re-attempting to create documents in a suitable format for faxing. I found the (unprinted) documents in a lingering unseen print queue. Having removed the documents, the uncontrolled disk writing has ceased.