Comment 80 for bug 8497

Having spoke to Matt Domsch, EDD kernel support's author, it seems it's the reading of the MBR that's typically causes booting problems when the drive being read doesn't exist. So there's no point doing just the MBRs and ignoring the advanced BIOS calls in the hope that it could be enabled by default. I've suggested to Matt that maybe 0x0040:0x0075 (BIOS's opinion on number of hard drives) could be read and used as a limit if it looks sane. Anything we can do to increase the number of machines EDD can run on without causing problems would help.

BTW, when grub is examing the MBR signatures and realises some aren't unique, it doesn't issue any diagnostic to that affect. It's possible the user would be happy to make them unique using, e.g. fdisk's expert's `i' command, but it should be drawn to their attention. Often, they're 0x00 or sometimes a drive is copied from another, resulting in two MBR sigs being the same for evermore.