can't install F7T3 off usb hard disk from DVD iso file with boot.iso

[Mark Schlegel]

I frequently install Fedora by downloading the DVD iso file
for installation via the boot.iso burned to a CD where the
DVD image iso file is on a usb hard disk (/dev/sdb1 )
On previous fedora's I usually used

boot:  linux askmethod

but for FC7T3 it seems you use the menu, and select the top
row "install or update..." and eventually you get to
"Installation Method".  I selected Hard drive and 
sdb1 as the device (correct for the usb drive with the 
DVD image on it). I used F2 to work into the drive
to select the DVD file.

The result is 

install exited abnormally [1/1]

tty3 shows:

trying to mount /dev/sdb1 as partition
mntloop loop6 on /tmp/drivers as /tmp/dpart/isos/f7t3/F-6.92-i386-DVD.iso fd 
is 11

error umounting: Device or resource busy


I'm guessing install from usb hard disk isn't set for the Test 3?

Mark