SanDisk 4GB Micro SD Card does not allow data

Hello,

I brought a SanDisk 4GB Micro SD card and used it in my mp3 player. Initially I copied few files and later when I tried to add more files or delete existing files. It doesn't allow me to do anything. I even tried to format the card but it did not allow.

I tried this using a different card reader but did not work either.

Please suggest a remedy if any knows how to solve this issue.

Regards,
S.V.Srikanth