How to boot from SD but run from US

dr> This is completely inappropriate for a Raspberry Pi, which does not

ot
 >  dr> boot like a PC using grub.

 > oeps.. my bad ..

 > So the maybe is: do not try this ..

 > Thanks for correcting me.

Actually I did read somewhere that it is possible. Technically you only need
to use the SD card to boot to init, so modify your /etc/fstab to change the
location of your root partition to the device of the USB drive.
(Theoretically, I haven't booted my Pi's in quite some time.. rapidly losing
interest in running a BBS) ..