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First off, I have no idea what I've done or am doing, so any help offered (which will be greatly appreciated) will have to be spelled out in great detail so I can understand what I need to do.

I've been using a computer lent to me by my friend and I was hoping to use an external hard drive I have to run Ubuntu (12.04) so I could have a more personalized space without putting his machine at any kind of risk. But now whenever I turn on the computer I get this message: "error: no such device: cb105e2b-b911-4e52-9dc6-504560332258e. grub rescue>" regardless of which OS I'm trying to load (the Ubuntu from the external HD or the Windows 7 from the internal).

I know there are plenty of questions on the same matter, but as I've stated I don't really know what I'm doing, so if I misunderstood what aspects of the answers were applicable to my situation, I could probably really mess something up, and I'd really rather not.

I don't know what details to include, sorry.

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This happened, because you probably installed the Ubuntu bootloader (grub) in the MBR of the internal HDD (or its bootable partition), yet the files it requires are installed on the external one. Further, the BIOS does not offer the external drive to the Ubuntu bootloader, so it doesn't find the required files.

If the system that was installed on the internal HDD was Windows Vista or 7, follow this guide for fixboot (and if that does not work also fixmbr): (there are similar guides for other Windows versions, yet I don't have the links at hand -> keywords for your own google search are fixmbr, fixboot, recovery console and of course your windows version)

If you want to use Ubuntu from an external HDD only, without touching the internal HDD, you must make sure, that the bootloader is also installed on the external drive (in that case, after installation you have of course to select the hard drive to want to boot from in BIOS). Somewhen during the install process (probably when partitioning your HDD) you get a drop down list of devices on which you can install Grub. It can of course be the case, that your BIOS does not boot from USB drives, meaning that there is no convenient way to install Ubuntu on an external drive without touching the internal one... I wouldn't be surprised if this really is the case on the computer you're trying to install Ubuntu on, since otherwise I don't see any reason why Grub shouldn't find its files on the external HDD right now...

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