Low Description
It looks like cassandra.utils.CLibrary.createHardLinkWithExec() does not check for any errors in the execution of the hard-link-making utility. This could be bad if, for example, the user has put the snapshot directory on a different filesystem from the data directory. The hard linking would fail and the sstable snapshots would not exist, but no error would be reported.
It does look like errors with the more direct JNA link() call are handled correctly- an exception is thrown. The WithExec version should probably do the same thing.
Definitely it would be enough to check the process exit value from /bin/ln for nonzero in the *nix case, but I don't know whether 'fsutil hardlink create' or 'cmd /c mklink /H' return nonzero on failure.
For bonus points, use any output from the Process's error stream in the text of the exception, to aid in debugging problems.