I set up a cold-to-frozen path about a month ago using RH7 and NFSv4. Everything worked the way it should. I was away for a week and this morning I found a ton of empty 'inflight-db_...' files in the frozen directories.
I looked at the splunkd.log and these error messages appear:
ERROR BucketMover - aborting move because recursive copy from src='/some/splunk/cold/db' to dst='/some/splunk/frozen/db/inflight-db...' failed (reason='Operation not permitted')
Splunk has write privileges to the directories. There is plenty of disk space available.