Unable to boot from external hard drive

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http://askubuntu.com – I'm new to ubuntu so please forgive any stupid mistakes. I'm trying to install ubuntu 11.10 to a 500 GB external usb hard drive, to dual boot with windows 7 (windows has already 4 primary partitions so that's the reason for the external drive). inside the installer (live CD), I choose the option "something else" and create a 20GB partition formatted as ext 4, with mouse point "/", a 400 GB partition formatted as ext 4 with mouse point "\home" and a 20 GB swap area (I have 8 gigs RAM). I then place the bootloader on the hard drive, dev/sdb. the installation goes on normaly, until it tells me (HowTos)