[ubuntu] grub failure - alloc magic is broken

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http://ubuntuforums.org – Any ideas for getting around this stumbling block, anyone? I'm testing quantal, and running several other Ubuntu installs, some on the main HD (sda) and others on a USB external drive (sdb). Since the update to grub2I've been having increasing problems with grub, not accessing some installs, etc. The latest manifestation is that booting from Precise, on my main HD, I get the error message: Code: alloc magic is broken at 0xb5b4aeb0. Aborted. Press any key to exit. Of course, pressing any key returns the same message, indefinitely. There is no grub> prompt, no grub rescue> promp (Hardware)