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I have: sda and sdb which partitions is forming mirror RAID

md0
md1
md2
md3

During update process, I was prompted where to install GRUB boot loader - on sda, sdb, md's or all together?

What choice is correct?

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Recommended way would be to install grub to a master boot record on each hard drive, that is sda and sdb.

That way if one hard drive fails you will be able to boot from another one.

Depending on your setup sometimes you may need to install grub to a specific partition rather than a master boot record. For example if you have another boot loader sitting on the master boot record and want to install grub to a partition then choose one of the md devices.

Similarly on EFI systems you should install grub to EFI system partition on both hard drives.

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  • Thanks Giedrius, I supposed so, but your third sentence is what concerns me. I was asked where to install GRUB during update process, after refusing to overwrite /etc/default/grub with new one, when I got message: "The Grub boot loader was previously installed to a disk that is no longer present or whose normally unique indentifier was changed for some reason..." and options to select (sda,sdb,md's). If that's telling something to you, any help would be life-saving to me. Jul 23, 2014 at 15:03
  • I suppose that this error can be triggered by a number of different things (e.g. disk change, partition changes, partition re-format). I didn't see this error before so I could only guess why you are getting it. However generally it's safe to install GRUB to master boot record unless you have some other boot loader living there. If you are not sure then most likely you don't have any other boot loader.
    – grekasius
    Jul 23, 2014 at 18:58

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