1) THE SD ram is not compatible...it happens... I have actually one that works on my camera but not with the PDA.. If it had worked before...then 2)
2) SD ram is faulty. If it works well with other PDA's..it points to 3)
3) your SD ram reader (contact) is oxidised....has no contact.... try eject/inserting it a few more times...and if your file browser cannot locate the SD..then I am afraid..you have a serious hardware prob...