Well, I solved the problem.
Among my 26,000 or so files, there were about 100 or so whose filename did not use standard ANSI encoding for accents and diacriticals. In those particular files, an “é” was made up of two separate characters. OSX doesn’t seem to like that. One way to verify this (since the filenames look normal in explorer) was to move the cursor over a file in JRiver. The path would then be shown and something like “Ma me`re l’oye” instead of Ma mère l’oye” would appear. This means that the normal looking è in explorer is made up of two characters.
I used the free utility “Bulk Rename” to change the offending characters. Even then it was a little difficult since I had to seek a bad character and copy it into renamer (typing a normal accented character didn’t work). I made a list of a bunch of them and bulk renamed the files while JRiver was opened. I reimported the files from the MAC version and it all went well except that I found I did have a couple of truly corrupt file which I then extracted. Hope that helps!