AWS Athena CTAS query failing, suggests emptying empty bucket
So I sprung for AWS Developer Support and asked this question. The response I got, which indeed fixed the error, was to create a folder within my external_location bucket. Not sure why this is necessary but apparently it is.
So, from shell:
$ aws s3 mb s3://<processed-data-bucketname>/processed_data/
(mb above stands for "make bucket").
Then updating external_location = 's3://<processed-data-bucketname>' in the query above to
external_location = 's3://<processed-data-bucketname>/processed_data/')