I found that on older versions of OZ, I was able to format my card despite there being no active partitions on it (so I formatted the device /dev/mmcda rather than a partition like /dev/mmcda1).
With 3.5.1 that changed and now appears to require a valid partition.
From a command prompt:
1) fdisk /dev/mmcda
2) p (print partition info). You probably won't see any. If there is a partition already, delete it
3) n (for new parion)
....p (primary)
....1 (make first partition)
....set start/end blocks based on what you want.
4) w (write partition)
Now format the new partition with mke2fs /dev/mmcda1
After it finishes, it may auto-mount the card, or you may need to pop it out and back in again.
One other issue which I had is that the default options in /etc/fstab did not work for my SD card. Edit this line:
/dev/mmcda1 /mnt/card auto defaults,sync,noauto,noatime,user,exec,suid 0 0
and change it to this:
/dev/mmcda1 /mnt/card auto defaults 0 0
(* note you may want to also keep the noatime option, but I'm not certain what this does)
Reboot and the card should now automount.
