Good instructions for getting an SD card loaded with the operating system and root filesystem: http://code.google.com/p/beagleboard/wiki/LinuxBootDiskFormat - So I followed the instructions for setting the geometry and partitioning, etc. Once I had that, I used a little script to take care of "loading it up". The script re-formats the two partitions, mounts them, and takes care of copying files and unpacking the images to the card. I wrote this "script" just to make it simpler to "start from scratch" after a new build... I'm assuming we're using the Open Embedded build environment, and the general automounter crud that goes on...
#!/bin/bash
# CONFIG - MODIFY THESE TO WORK ON YOUR BUILD
SD_BOOT=/dev/sdc1 #<<< YOU MUST SET THIS CORRECTLY OR YOU WILL BE UNHAPPY!
SD_ROOT=/dev/sdc2 #<<< YOU MUST SET THIS CORRECTLY OR YOU WILL BE UNHAPPY!
DEV_PATH=/stuff/angstrom-dev/deploy/glibc/images/beagleboard
MLO=$DEV_PATH/MLO-beagleboard
UBOOT=$DEV_PATH/u-boot-beagleboard.bin
XLOAD=$DEV_PATH/x-load-beagleboard.bin.ift
UIMAGE=$DEV_PATH/uImage-beagleboard.bin
ROOTFS=$DEV_PATH/console-image-beagleboard.tar.bz2
# PROGRAMS
UMOUNT=/bin/umount
MKFS_DOS=/sbin/mkfs.msdos
MKFS_EXT3=/sbin/mkfs.ext3
GMOUNT=/usr/bin/gnome-mount
CP=/bin/cp
# Make sure partitions are NOT mounted
echo Making sure SD card is unmounted
$UMOUNT $SD_BOOT
$UMOUNT $SD_ROOT
# Format the BOOT partiton as FAT32
echo Formatting SD card partitions
$MKFS_DOS -F 32 $SD_BOOT -n BOOT
$MKFS_EXT3 -L LINUX $SD_ROOT
# Mount the images using gnome mount
echo Mounting partitions
$GMOUNT -d $SD_BOOT
$GMOUNT -d $SD_ROOT
# Copy the MLO file -- must be first file/blocks of the FAT partiton for it to work!
echo Copying Boot Files...
$CP $MLO /media/BOOT/MLO
# Copy the other boot files
$CP $UBOOT /media/BOOT/u-boot.bin
$CP $XLOAD /media/BOOT/x-load.bin.ift
$CP $UIMAGE /media/BOOT/uImage.bin
# Extract
echo Extracing Root Filesystem... this will take a few minutes
cd /media/LINUX; tar -xjf $ROOTFS
# unmount so you can use it!
echo Unmounting SD card
$UMOUNT $SD_BOOT
$UMOUNT $SD_ROOT
echo Done!