Just to confirm what others are saying, the iPAQ 3955 and 3970 only read/write MMC and the original "Secure Digital" (SD) cards, which topped out at 1 GB. I believe there was a firmware update that allowed use of certain 2 GB cards. SDHC, SDXC, etc., cards with larger capacity don't work. There is also a formatting issue, I don't think the newer exFAT format is supported in any capacity card.