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Posted: Fri Nov 18, 2011 3:42 am
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Technically, you're setting the offset of the last partition to an aweful lot of GBs. The BIOS then thinks there's no partition table because it's broken, and that the USB device therefore contains a floppy (image) instead.
You might probably notice that the BIOS will try to boot each configuration differently. When it thinks it deals with an FDD it will set DL to zero, while it will set DL to 0x80ish when your partition table is indeed valid.