Just a quick note, I don't even unplug my external drive. If you simply turn the drive off and reboot it will return to the internal drive. That is if yours has a power switch, which mine does. Just thought I would put that out there. It is a hole lot easier than unplugging the eSATA cable from the back of the HR20.
So, is the inverse true as well ? I mean instead of trying to time yanking the power cord (per BMoreRavens post #14 above) when you restart the HR20 (lights out for moment).....can you just plug in the external drive while HR20 is running, power up external HDD, then restart (RBR or Menu Restart) the HR20 and it will see external drive ?