02.02.2016 18:36, Carlos E. R. пишет:
I used "Boot Info Script 0.72" (https://github.com/arvidjaar/bootinfoscript/raw/master/bootinfoscript) to obtain information on the boot process.
Could you send me privately RESULTS.txt and GPT table (first 32K of disk)?
Notice that there is no partition marked as boot, and that there is a "BIOS boot" partition, very small.
The MBR has this:
=> Syslinux GPTMBR (4.04-5.01) is installed in the MBR of /dev/sda.
...
Now, what I don't understand is, how is BIOS booting this disk. Obviously it has to read the MBR, and this boots sda2. How does it know it has to boot this partition?
If "it" means BIOS, it does not know anything. It loads the first sector and jumps to it. Now gptmbr code of Syslinux looks for GPT partition that has "legacy BIOS bootable" attribute and jumps to it. You can see flags using gdisk `i' command (display detailed partition information). Leacy BIOS bootable is bit 2. This changed relatively recently (6 years ago :); before gptmbr had boot partition GUID encoded.
Notice that there is no partition marked as boot, and that there is a "BIOS boot" partition, very small.
This is irrelevant in your case but may help if you decide to install GRUB2 in MBR. P.S. of course not every BIOS is equal and there are known cases when BIOS won't jump to boot code unless it sees legacy MBR active partition. And of course there are firmwares that go amok when they see PMBR with Active flag ...