You must unmount either the partition or the mount point, so either of these will work (and you only need to use one):
umount /dev/mmcda1
umount /mnt/card
I don't use pdaXrom, so check the output of 'mount' to see that you're unmounting the right thing.
If you want to create partitions then use the instructions linked above. Note that to create partitions you must pass the device, not the partition (or mount point), to fdisk, so:
fdisk /dev/mmcda
(note that there's no 1 on the end).
To format in your current state (first and only partition) do:
mkfs.ext2 /dev/mmcda1
(swap the 1 for a 2 once you've made two partitions, etc.)
Si