Of course I selected the files first. I also tried to drag and drop, without effect.
To be specific, I right click on a line/number/album in the audio list of the music on my pc, go to SEND TO, choose the location of the music player and the dialog appears "LOADING HANDHELD". That takes a lot of time. After minutes there is the window in the bottom left corner that says SYNC and the text Sync 19844 203 Gb over capacity.
I really don't know what I am missing here. Clicking on SYNC will commence the process of syncing the entire music player. That is not what I want.
When I follow your suggestion, to drag and drop from a split screen, it does not work either. I go to split view, choose the music player, so that the left pane fills with the entire list of all my music on the player, I click on audio from the sidebar in order to do a search on the number or album I want to copy from my pc to the player, and the contents of the music player in the left pane is replaced by the result of the search. So having split view had no effect. Dragging the result to the music player only led to the appearance of the text IN QUEUE before the number I wanted to copy. But there is no copying being done.
I have the Pioneer XDP-100R with two microsd slots, each with 200 gb largely filled with music.
UPDATE: Copying a music file to a location that JRiver labelled as not synced before did work. Once. The second time I tried to copy a song the result was zero as before. It appeared in the list with the text IN QUEUE, without any copying being done. It seems there is a link with syncing, though I did not do any sync operation. Dragging the queued song to the device did copy the one song. But there should be a simpler way than this.
UPDATE 2: Problem resolved. In the sync options I unchecked AUDIO. And so I only saw the contents of the device.