[all variants] Loading encryption key file from encrypted USB drive on boot

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http://ubuntuforums.org – This is an interesting little use case I've dreamed up, and I'd like to know how I might go about it. I have 2 drives in my system, sda and sdb. sda currently contains /boot and /, the latter of which is encrypted. sdb contains /home, and is encrypted using a key file stored in /root. sda is an SSD. I'm planning on upgrading soon to 13.04, and in the process I'm going to reformat sda, and install unencrypted, for performance reasons. Which leaves me with the conundrum of what to do with the key file for sdb. My first thought was, to put it on a USB drive. Then I realized, that if (Hardware)