What file system is on the external drive?
Mac OS Extended (Journaled).
Running some more tests, I can see that the xmp file is indeed written to disk when assigning a rating, but the rating is not visually displayed below the thumbnail. This test was done with build 17719.
<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="XMP Core 5.1.2">
xmp:CreatorTool="NIKON D4 Ver.1.10 "
UPDATE: Another strange thing: testing on a different folder on the same drive, ratings are displayed correctly. Seems random - works with some folders, not with others. All folders have Read & Write privileges. Seems like the issue is related to reading the xmp files and/or updating the thumbnail view.