After spending a whole night trying to get the latest OS2008 to boot from my MMC and failing miserably, I've decided to go back to booting from flash

However, I'm now stuck with a 16GB SDHC that Windows only recognizes as 14GB since there's a LINUX partition in there.

How do I repartition the SD back to a full FAT system?

I tried

sudo gainroot
umount /media/mmc2
sfdisk /dev/mmcblk0 -uM
1,,0C
<enter>
<enter>
<enter>

Then I get a message saying

Warning: No primary partition is marked bootable (active). This does not matter for LILO, but the DOS MBR will not boot this disk.

At which point I chickened out.

Rather than risk corrupting my SD beyond repair, what is the correct command to use to get this SD back to a full FAT system?