MBR vs GPT, which takes priority by GRUB2?

[Chris Murphy]

I have a case where a linux distribution defaults to creating GPT disks, on 
BIOS based hardware. Subsequent re-partitioning by Windows Vista or 7 causes 
the GPT structures to remain untouched and intact, with only LBA 0 modified 
such that a valid legacy MBR is created. This condition is also reproducible by 
util-linux fdisk.

When a disk contains a valid legacy MBR *and* an otherwise valid GPT, which 
does GRUB2 honor?

From UEFI 2.3.1 Errata A. 5.3.2 says:If a GPT formatted disk is reformatted to 
the legacy MBR format by legacy software, the last logical block might not be 
overwritten and might still contain a stale GPT. If GPT-cognizant software then 
accesses the disk and honors the stale GPT, it will misinterpret the contents 
of the disk. Software may detect this scenario if the legacy MBR contains valid 
partitions rather than a protective MBR (see Section 5.2.1).

So it seems clear that the valid legacy MBR should be honored, and the GPT 
ignored.

Chris Murphy