I am currently trying to update the firmware on my GP2X and also use a different U-boot. Since my Card currently is not firmware complatible, the best answer I got is to completely remove the partition on the card or/and reformat in fat32 (0B) using something like fdisk in llinux.
Control Panel->Administrative Tools->Computer Management doesn't allow me to remove or modify SD card partitions
I am alos having trouble trying to mount my multi card reader under linux as well..