Further to this, it also seems that it is not possible to set a checkpoint directory on an external location where the principal has write permission to the external location. 

When we try:

spark.sparkContext.setCheckpointDir("s3://bucket/path")

we see:

com.amazonaws.services.s3.model.AmazonS3Exception: Access Denied;

(I know its not a permissions issue, because I can read and write dataframes to the same path on the same UC cluster).

We've also tried setting the checkpoint directory through the spark configs like this:

spark.conf.set("spark.checkpoint.dir", "s3://bucket/path")

But we get:

[CANNOT_MODIFY_CONFIG] Cannot modify the value of the Spark config: "spark.checkpoint.dir".
See also 'https://spark.apache.org/docs/latest/sql-migration-guide.html#ddl-statements'. SQLSTATE: 46110
File <command-5849427671817506>, line 1

Both attempted on DBR 15.4, dedidated cluster.

I am shocked. Is it not possible to use checkpoints on UC???? There must be something I am overlooking.