This issue is due to osxfs currently missing a statfs implementation. See https://forums.docker.com/t/cant-start-elasticsearch-with-mounted-data-directory-f-stat-v-fs-family-of-functions/8705. The Cassandra 2.1 error message is misleading but if you run Cassandra 2.2, you will see:
cassandra_1 | Exception (org.apache.cassandra.exceptions.ConfigurationException) encountered during startup: Unable to check disk space available to /var/lib/cassandra/commitlog. Perhaps the Cassandra user does not have the necessary permissions
cassandra_1 | org.apache.cassandra.exceptions.ConfigurationException: Unable to check disk space available to /var/lib/cassandra/commitlog. Perhaps the Cassandra user does not have the necessary permissions
cassandra_1 | at org.apache.cassandra.config.DatabaseDescriptor.applyConfig(DatabaseDescriptor.java:522)
cassandra_1 | at org.apache.cassandra.config.DatabaseDescriptor.<clinit>(DatabaseDescriptor.java:119)
cassandra_1 | at org.apache.cassandra.service.CassandraDaemon.activate(CassandraDaemon.java:491)
cassandra_1 | at org.apache.cassandra.service.CassandraDaemon.main(CassandraDaemon.java:625)
cassandra_1 | Caused by: java.nio.file.FileSystemException: /var/lib/cassandra/commitlog: Function not implemented
cassandra_1 | at sun.nio.fs.UnixException.translateToIOException(UnixException.java:91)
cassandra_1 | at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:102)
cassandra_1 | at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:107)
cassandra_1 | at sun.nio.fs.UnixFileStore.readAttributes(UnixFileStore.java:111)
cassandra_1 | at sun.nio.fs.UnixFileStore.getTotalSpace(UnixFileStore.java:118)
cassandra_1 | at org.apache.cassandra.config.DatabaseDescriptor.applyConfig(DatabaseDescriptor.java:517)
cassandra_1 | ... 3 more
and so you are correct and it is exactly the same issue as https://forums.docker.com/t/mounting-volume-with-mongodb/13606/2 which we are actively working on.
Thanks for the report and for participating in the Docker for Mac Beta!
David