In case someone else happens upon this I have found that mdadm >=
v2.6.2 cannot add a disk to a degraded raid1 array created with mdadm
< 2.6.2.
I bisected the problem down to mdadm git commit
2fb749d1b7588985b1834e43de4ec5685d0b8d26 which appears to make an
incompatible change to the super block's 'data_size' field.
--- sdb1-sb-good.hex 2007-12-12 14:31:42.000000000 +0000
+++ sdb1-sb-bad.hex 2007-12-12 14:31:36.000000000 +0000
@@ -6,12 +6,12 @@
0000050 60d8 0077 0000 0000 0000 0000 0004 0000
0000060 0000 0000 0000 0000 0000 0000 0000 0000
*
-0000080 0000 0000 0000 0000 60d8 0077 0000 0000
+0000080 0000 0000 0000 0000 60d0 0077 0000 0000
Which trips up the "if (rdev->size < le64_to_cpu(sb->data_size)/2)"
check in super_1_load [1], resulting in:
mdadm: add new device failed for /dev/sdb1 as 4: Invalid argument
--
Dan
[1] http://lxr.linux.no/linux/drivers/md/md.c#L1148
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