Files on media card not showing on Mac

I've had this problem and I'm sure its just a simple setting....

Whenever I plug in an mp3 player or a media card through a card reader, I can manage the files on the mac.

When I delete the files on the card or mp3 player from the mac, it no longer shows them (even when I plug it in at a later date), but they are still on the card. Under properties of teh card and player it is showing they are both full, but when I open the devices, it shows nothing on them. All the files are showing on a pc and still on the device.

The problem I'm running into is that I want to change the playlist but all the songs that I previously had on the player that I thought I erased though the mac are still on the player but not showing on the device when its plugged in (I run into the same thing when I plug in the media card through the card reader).

Any and all help greatly appreciated.

Matt