Thanks for your patience on this, Kochin!
I have tried 3 separate USB sticks, all with same results: same "additional required dependencies" errors (?) on installing; then booting only into the Pogoplug NAND (evidenced by "Pogoplug:~$"), not the USB drive.
The dependencies issue seems more of a debian/doozan problem than my USB drive. But the refusal to boot from the USB drive is a puzzle. Your fdisk setup instructions did not specify setting the sd1 partition as bootable, but I did this anyway just because it seemed a good idea in face of my bootup issues. (BTW, that setup section could use more fleshing out for newbies to linux such as myself, IMO).
So my distilled question: is the unresolved dependencies during the installation a real show stopper, or can I safely ignore this and proceed to focus on forcing a boot to USB? Or maybe installing Debian on NAND?