Bug#743512: Also seen in grub-pc 1.99-27+deb7u3

[Robert L Mathews]

I can confirm this report. I occasionally do what the original reporter
does: replace an entire set of disks on a live server using mdadm --fail
/ mdadm --remove / [physical hot swap] / mdadm --add / grub-install, and
the identical problem happened to me.

In my case the disks are a three-disk RAID 1 array rather than RAID 10,
and I partitioned them with "parted" rather than using "dd".

I replaced the first two and grub-install worked with no trouble:

# grub-install /dev/sdc

# grub-install /dev/sdb

But the third gave me:

# grub-install /dev/sda
/usr/sbin/grub-probe: error: no such disk.
Auto-detection of a filesystem of /dev/md0 failed.

# grub-install --recheck /dev/sda
/usr/sbin/grub-probe: error: no such disk.
Auto-detection of a filesystem of /dev/md0 failed.

... with the other symptoms identical.

To my astonishment, running this:

# mdadm --examine /dev/sda2

... fixed it; "grub-install /dev/sda" then works without errors.

Clearly "mdadm --examine" has a mysterious helpful side-effect, the
nature of which is puzzling.

However, I can't reproduce the original problem on demand; this has
happened to me only once when doing this on about half a dozen
physically identical (in terms of hardware) servers.

-- 
Robert L Mathews