I was finally able to use a USB flash drive to store my root filesystem and do the rest of the RPi boot. this looks great! My USB drive partition is around 16GB, but only 3GB is used. My micro-sd card is 8gb.
I may want to clone the modified root partition with all my data back onto the SD card and use on another pi.
My question is: is there any difference between using 'dd' vs a copy all? Since my USB drive partition size is 20GB (3GB used), the dd image will also be that size. Unless I resize the partition down to 8GB, it won't fit onto the SD-card. So why don't I just remove everything from the old root partition on my SD-card, and just copy paste all files from the USD drive root partition there? Is there anything wrong in this?