I think because there are actually 4 drives competing for real estate on your SDcard. 2 gigs of your rated 4 are on the external card, so there is a little booter script in the vold.fstab file that splits up your card:
# Internal sd aka emmc
BOARD_HAS_SDCARD_INTERNAL := true
BOARD_SDCARD_DEVICE_PRIMARY := /dev/block/mmcblk1p1
BOARD_SDCARD_DEVICE_SECONDARY := /dev/block/mmcblk1k1
BOARD_SDEXT_DEVICE := /dev/block/mmcblk1p2
I don't know how to fix this, but when I do I will let everyone know.
If you place something here: /mnt/sdcard/_ExternalSD/
you can see it via ur pc,
but if you put it here:
/mnt/sdcard/
it will unmount from the system and place all its data into EMMC when the usb hits voltage.