[G Tablet Q&A, Help & Troubleshooting] rajeevvp:
That is not quite accurate. A quick FYI might be helpful here:

The gTablet can have up to 2 SD cards, one internal--ie built into the board--and one external, which the user can purchase and insert into the MicroSD card slot.

The Linux kernel understands these 2 SD cards as follows:

/dev/block/mmcblk3 - this is the internal SD card device, which is always present.

/dev/block/mmcblk2 - this is the external SD card device, if you have it.

The internal SD card is 16GB (unformatted) in size. ViewSonic's ROM, and all the other 3rd party ROMs for the gTablet, (generally) create 2 partitions on this internal SD card. The partitions have certain standard sizes, are formatted as 2 different filesystems, and are mounted at specific places depending on the ROM.

Internal SD card (raw ie. unformatted sizes):
Partition # Linux device name Size Mounted at Filesystem type
1 /dev/block/mmcblk3p1 ~14GB /mnt/sdcard VFAT
/mnt/emmc
2 /dev/block/mmcblk3p2 ~2GB /data ext3

Froyo-based ROMs, like ViewSonic's default, TnT-Lite and VEGAn-5.1.1 mount the first partition at /sdcard (or /mnt/sdcard--both are the same, one is a symlink to the other). The Gingerbread-based ROMs like CyanogenMod mount the same partition at /mnt/emmc (or, /emmc). VEGAn-Ginger is an exception--it uses the Froyo convention.

If you have an external SD card, it, usually, will have just 1 partition on it and it will be formatted as VFAT (aka FAT32). If this SD card has another partition on it and it is formatted as ext3, then we will have this situation (at least on CyanogenMod 7.x):

External SD card:
Partition # Linux device name Size Mounted at Filesystem type
1 /dev/block/mmcblk2p1 [I]n1[/I]GB /mnt/sdcard2 VFAT
/mnt/sdcard
2 /dev/block/mmcblk2p2 [I]n2[/I]GB /sd-ext ext3

The Froyo convention for the first (or only) partition on the second SD card is /mnt/sdcard2. In GB ROMs, that partition is mounted at /mnt/sdcard. I'm not sure what Froyo-based ROMs do to the second partition on the external SD card, but, CyanogenMod--a GB-based ROM--mounts it under /sd-ext.

2 partitions at most, on the 2 SD cards, are what the ROMs all like. You can have more partitions, but, the automounter program--vold--won't like it and the extra partitions will not appear under Settings > Storage for easy mounting and unmounting. You'll have to resort to the command line for that. (In fact, I don't even know how the partitions will be assigned to the limited no. of standard mount points when you have more than the standard no. of partitions on the SD cards.)

The same caveats hold for non-standard filesystem types. Vold likes VFAT, ext3 and ext4 (maybe even ext2) filesystems because it can sanity check these partitions using the default fs checker programs supplied with the ROMs. Other filesystems, it can't check before mounting and so it will ignore them--even if the kernel supports the filesystems (through loadable modules).

The other partitions on the gTablet, /system, /cache, ... are all on a NAND flash chip on the system board. These have a different filesystem on them called yaffs2, and they show up as a different class of devices: /dev/block/mtdblockn. The nvflash program only operates on these directly-accessible NAND flash chips. Therefore, it cannot erase any data on any of the SD cards. You must use ClockworkMod's menu options to wipe or partition SD cards.

You can, of course, also use the command line. For example, to completely wipe the internal SD card, you can boot into CWM, connect to it using adb and then type

busybox dd if=/dev/zero of=/dev/block/mmcblk3

at the prompt, or, you can wipe only the second partition of the external SD card with:

busybox dd if=/dev/zero of=/dev/block/mmcblk2p2

In fact, if you're very brave, you can even run the same commands while in Android--after unmounting the filesystems. (Don't mess with /data, though, while the system is running.)