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">
<rdf:RDF xmlns:rdf="
http://www.w3.org/1999/02/22-rdf-syntax-ns#">
<rdf:Description rdf:about=""
xmlns:xmp="
http://ns.adobe.com/xap/1.0/"
xmlns:photoshop="
http://ns.adobe.com/photoshop/1.0/"
xmlns:photomechanic="
http://ns.camerabits.com/photomechanic/1.0/"
xmlns:aux="
http://ns.adobe.com/exif/1.0/aux/"
xmp:CreatorTool="NIKON D4 Ver.1.10 "
xmp:CreateDate="2016-03-08T09:06:04.70"
xmp:Rating="4"
photoshop:DateCreated="2016-03-08T09:06:04.70"
photomechanic:ColorClass="0"
photomechanic:Tagged="False"
photomechanic:Prefs="0:0:4:004522"
photomechanic:PMVersion="PM5"
aux:ImageNumber="4522"/>
</rdf:RDF>
</x:xmpmeta>
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.
Jarle