M3u playlists are the format you want (not .pla). These get stored in the Music folder, and not the Playlist folder on the player. So you can create a playlist of songs located on an external memory card, store the playlist file in the same Music folder (as the song files themselves) on the card, and they should work reliably regardless of which player the card is inserted in.
Note this won’t work with songs from both memory locations (in the same playlist). For that, you need to use .pla format.
Here is a step-by-step tutorialI wrote some time ago here to create playlists using Winamp. While I use a PC, there is a Mac version of Winamp as well. It may help do what you want. The procedure should be the same, or at least similar enough for you to adapt it.
Hope this helps. 
