Hi,
I've raid1 system with 2 devices: sda and sdb.
Now I need to replace /dev/sda.

I know that default Debian installer puts GRUB only on first device (that's very strange!!).

I've do this:

# grub-install /dev/sdb

No error reported.

Anyway, after --fail and --remove of /dev/sda1, sda2 and sda3 from respective md devices, and after physically remove sda, I can't boot my system, and I receive

"Grub Hard Disk Error".

How can I solve this?
Thankyou!

Did you replace the first drive or do you now only have the one drive, the one which was sdb?
If you only have one drive now, boot any Linux Live CD, open a terminal and log in as root user and run the command to get partitions:

fdisk -l(Lower case Letter L in the command)

Check to see partition name(s), sda, sdb, ? Mount the partition where your boot files are and look at the grub.cfg file to see if they are named correctly, pointing to the same drive/partition as you see in the fdisk output. If your fdisk output shows sda and the grub.cfg entries are pointing to sdb partition, that could be your problem. Without any specific drive/partition or boot file information there is not much we can do here other than guess.

Now, only sdb is attached.
And the system is unbootable.
Anyway, if I attach sda too, all is fine.
If I use LiveCD (Fedora 17 in my case), with only sdb attach, I see "/dev/sda1".
It's hard to mount, as this is raid device, not standard partition.
Should I create, from LiveCD, a degraded raid1 array and try?

Probably what you need to do is look at the grub.cfg file on the drive, the one that was formerly sdb but now shows as sda as you have only one drive. The grub.cfg entries may be pointing to sdb when they should be pointing to sdb. Specifically, look for the the:

set root line as above. The example above is pointing to sda1 so if you have that pointing to (hd1,msdos1) it won't work.

If it's listed as sda1 then you probably need to check ftsab & make corrections there.

Hi, I'm a bit confused.
If it's correct to see "sda" (as I've only a disk), when I'll add the new disk.. it will be "sdb"? At this point I'm afraid of not knowing what is really sda.

What are your intentions? Are you going to have the Debian system on your new sda drive? Do you also have Debian installed on the sdb drive? If you are replacing what was the sda drive with a new one, install Debian on it and try to boot. I'm not really clear on what your intentions are.

Please read my first post.
I've a server with raid1 (sda and sdb).
As you know, Debian installer puts GRUB only on sda.
Now, sda is failing and I need to replace it.
If I remove sda, and kept only sdb on server, but system isn't able to boot. Because simply GRUB isn't in sdb MBR.

So I reinstalled both disks on server, do a "grub-install /dev/sdb" with no error.
Again, I removed sda but I got "Grub Hard Disk Error".

At this point I've no idea on how really replace sda.

Hi EDDY1, before reinserting sda ​​drive, I run the raid commands: mdadm --fail /dev/sdaX and mdadm --remove /dev/sdaX, so I have not had the problems described in your url.
Thankyou!

I was assuming that the information from post #2 was relevant, if not I apologize.