Matthew Garrett continues
into the subtleties of booting Linux with EFI. "GPT, or the GUID Partition Table, is the EFI era's replacement for MBR partitions. It has two main advantages over MBR - firstly it can cover partitions larger than 2TB without having to increase sector size, and secondly it doesn't have the primary/logical partition horror that still makes MBR more difficult than it has any right to be. The format is pretty simple - you have a header block 1 logical block into the media (so 512 bytes on a typical USB stick), and then a pointer to a list of partitions. There's then a secondary table one block from the end of the disk, which points at another list of partitions. Both blocks have multiple CRCs that guarantee that neither the header nor the partition list have been corrupted. It turns out to be a relatively straightforward modification of isohybrid to get it to look for a secondary EFI image and construct a GPT entry pointing at it. This works surprisingly well, and media prepared this way will boot EFI machines if burned to a CD or written to a USB stick.
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