Follow these steps to restore a dump.rdb into a Redis master that is running in append-only mode. For some reason this is not documented very well. The master in this case had an issue and was unable to write to disk. I was able to get the dump.rdb from a slave.
Step 1 - Obtain a dump.rdb & check the size for later comparison
redis-cli -p $PORT save redis-cli -p $PORT dbsize
Step 2 - Stop the master & all slaves