1) Format a USB Flash Drive with EXT4 File System
2) Decompress rootfs.tar.gz into USB Flash Drive
cp rootfs.tar.gz /media/USBDrive
cd /media/USBDrive
tar -jxvf rootfs.tar.gz
rm rootfs.tar.gz
3) Now we will tell the board to boot from USB

cp ~/RowboatTools/am37x/mk-bootscr/mkbootscr ~/RowboatTools/am37x/mk-bootscr/mkbootscrUSB

vi  ~/RowboatTools/am37x/mk-bootscr/mkbootscrUSB

change root=/dev/mmcblk0p2 rw rootfstype=ext4 rootwait to root=/dev/sda1 rw rootfstype=ext4 rootdelay=30 rootwait
4) Build boot.scr
cd ~/RowboatTools/am37x/mk-bootscr/
./mkbootscrUSB
5) Copy boot.scr into SD Card – Boot partition
cp boot.scr /media/boot
6) Unmount all drives and boot with SD Card and USB Flash Drive inserted
Later you can try formating the “rootfs” partition in the SD Card and restart the board. Don’t erase the partition, just the content.
You will find that running Android on Beagleboard from USB may be, by far, faster than from SD Card.