Looks like you're doing very well. Since I've never tried the usb
thing and wouldn't want to give you bad advice, let's just call this
"throwing it out there".
In the web interface, look at the menu under Packages > Alt-F
It appears I've installed Alt-F in one place and, at some point
copied it to another. Both are boot enabled and one is active.
There is a combo box to choose another filesystem, copy Alt-F there,
boot-enable, and activate. I'm guessing/vaguely remembering that,
if Alt-F packages have not yet been installed, this menu section
would give you the options to work with the usb drive.
I've got 4 or 5 DNS-* running Alt-F and, for a variety of reasons,
my grasp of exactly what happened, when, where, should be an object
of a bit of skepticism. Mull it over, apply your apparent powers of
critical thinking, and, if necessary, better minds might rescue you
from making the wrong step, given time.
Good luck!
Rolf