MP3 player can no longer read CF card with XP and WMP 10.

I just built a new sysytem. Went from Intel Celeron with ME, to AMD Sempron
with XP Pro (SP2). I had an mp3 player that uses CF card and with my old
system I only used winmx for downloading songs and musicmatch for playback,
so until now I've never used WMP. When I plug my card into my card reader
and attach it to usb WMP automatically begins to read and play the files
dropped on the card, but when I took those songs off to replace them with
other mp3's my mp3 player just says 'no mp3 file in :a/'. I've tried
reformatting the card, and reformatting wma files into mp3 files, in case it
was a compatibility problem with wma files, but I'm still at a loss as to
what else to try. I'm not so quick to convenience myself that my mp3 player
is too outdated to be compatible any longer with XP, but is this a
possibility? It's about 4 years old, a 'Super Mello' mp3 player.