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I have a 1TB external HDD, split up into 4 partitions. One of which is a primary partition I'd like to boot from. Unfortunately, I am unable to mark it as active and whenever I try to play around with the BIOS, it still doesn't work. This partition does have an OS installed. How can I boot from it?

UPDATE: If it makes any difference, my computer supports UEFI.

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migrated from Sep 18 '11 at 22:21

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Could you be more specific on "an OS"? – grawity Sep 18 '11 at 22:34
it has the windows 8 installer – sddhhanover Sep 18 '11 at 22:42
You need to explain what exactly you are trying to accomplish, boot from W8 to install it?, or actually run an OS from USB hard drive? – Moab Sep 18 '11 at 22:50
I am trying to install Windows 8 from the external HDD partition onto my computer. – sddhhanover Sep 18 '11 at 22:52
How are you "unable" to mark an entry in an (old-style) partition table as "active"? Show us, in your question, what the behaviour of diskpart's active command was when you tried it. – JdeBP Sep 20 '11 at 22:21

A partition must be marked as Active in order to be bootable.

USB drives are bootable. You'll need to make sure there is a valid boot sector on the USB drive and properly configured for booting.

If you're having issues booting from USB you'll have to remove the drive from the enclosure and connect it directly to the PC.

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Since the HD itself is a 3.5", it won't fit in my computer. How do I configure it for booting? – sddhhanover Sep 18 '11 at 22:41

You may use one of the smaller Linux distros, such as Gparted's live CDs, Slitaz or Puppy, install them with Unetbootin, and use the GParted tool to inspect whether the sector is marked as bootable or not. You can mark it as bootable from there.

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