When using Debian Squeeze with GRUB v2 together with a software mirror built via mdadm (for example, right during OS installation) - everything is fine, but the system won't boot if you pull out the first hard drive.
In other words, it turns out that when using Software RAID-1 Mirror in Debian Squeeze, the OS can only boot from the first disk. If that one fails, the data on the second disk is of course still there, but the system will no longer be able to boot from it.
Nonsense, sure, but that's the little surprise the Debian developers threw at us.
Let's fix this problem. To do that, we'll also install GRUB on the second hard drive.
1) Run:
$ sudo dpkg-reconfigure grub-pc
2) In the window that opens, for the "Linux command line" question, leave what's already there (by default - nothing).
3) Next - for the "Linux default command line" question - again leave the suggested value (by default - "quiet").
4) Next, select both hard drives on which you want the bootloader installed (for example, /dev/sda and /dev/sdb).
Attention! You do not need to select the md disks!
5) Open the /etc/default/grub file for editing and uncomment the line:
GRUB_TERMINAL=console
after which save the file.
6) As recommended in this file - rebuild the GRUB config:
$ sudo update-grub
And that's it - now the system will be able to boot from either hard drive, no matter which one survives.
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