alternate install to USB bootloader issues using lvm encryption

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http://askubuntu.com – I am trying to use the alternate installer (12.04.1 i386) run from a cd to install onto a usb thumb drive (to have a portable installation). I am using the guided lvm install. I get to the part where it asks to install the bootloader into the windows 7 mbr on the main hard drive so of course i say no, but then it asks where instead i would like to install it, i type in the name of the unencrypted volume on the thumb drive /dev/sdb1 but it gives an error and will not install. am i doing something wrong? any help is appreciated as i am about to have a cerebral hemorrhage. (HowTos)