I'm not sure if you are still looking for an answer,
(been a while since you posted). I'm not an expert, but
here is my best guess. I bought a Rio Forge player that
uses an SD expansion card. In most players built today,
they are just that, an expansion of the built-in memory.
When the built-in memory gets filled, the overflow goes
to the card. Therefore, it is probably not compatible
with other players. Most new players also come with their own software to load the songs. There is probably some other formatting on the cards that is particular to each brand of player.
If your old SD cards do not work with a new player, they can be re-formatted and then re-used.
Hope this helps.