Coming back the following day, I now see the files in Windows Explorer. Not sure what changed, but they are there now.
Unfortunately, the player (Sony Walkman NW-A306) doesn't recognize the newly synced files.
And the reason for that is it doesn't support .ogg files, so it ignores them. Prior to moving to JRiver, I used iTunes AAC at 256k VBR to rip, and the .m4a files worked in but MC and the player. Since MC doesn't seem to rip AAC, I've been ripping with Vorbus, but those .ogg files won't work on the player.
I either need to re-rip the most recent CDs, convert them in place, or set MC to convert on export. I just need to figure out which file format to settle on. The spec for my player is here:
https://helpguide.sony.net/dmp/1301/v1/en/contents/TP1000752534.html Suggestions for formats that are compatible with both MC and the player cheerfully accepted. Thanks.