I have Cingular 3125 windows smartphone. When I bought it I also got Kingston 1GB MicroSD flash card to stash my MP3 files on it. That didn't work well. The behavior was strange: all files and folders on the storage card were accessible immediately after phone was turned on, but some time later only folders in the storage card's root were shown by the phone's file explorer or Windows Media Player library - all other files and folders seemed missing until phone was powered down and then turned back on. I replaced the card with Sandisk, which worked fine - until something happened and all files on the card got corrupted or missing. I had to re-format the Sandisk card and I formatted it as FAT, a.k.a. FAT16. To my astonishment, it has started to behave just like my old Kingston card. I reformatted it again as FAT32 and it has started working fine! So here you go: format your storage card as FAT32 for using it in your Microsoft Windows Mobile Smartphone.