Rather than messing with curl, I went the long way and installed Kibana. It takes a little while to unzip. Run it in a very similar way to Elastisearch (bin\kibana.bat). Go to http://localhost:5601/app/kibana#/dev_tools.
Paste in the solution from here:
PUT /<yourindex>/_settings { "index.blocks.read_only_allow_delete": null }
You may have run into this because you are close to the disk space limits. Rather than modifying those through Kibana, you can add them directly into elasticsearch\config\elasticsearch.yml.