degraded raid 1 help

[Bruno Wolff III]

On Mon, Jan 23, 2006 at 16:32:07 -0800,
  Bazooka Joe <fastfish at gmail.com> wrote:
> I have 2 sata 300 gb hard drives mirrored - all partitions are mirrored.
> 
> this weekend the home (md6) partition lost one of its members (sda8) but sda
> did not fail only one partition.
> 
> So I assume i need to hot add back in sda8 to md6
> 
> and from the mdadm man page i got
> 
> # /sbin/mdadm /dev/md6 -a /dev/sda8
> mdadm: Cannot open /dev/sda8: Device or resource busy
> 
> am I doing something wrong???  This is the first time i have used mdadm to
> manage raid.
> 
> do i have to unmount md6??

Its been a few months since I had to do this, but my memory is that you have
to fail or remove the bad partition, before adding it back.

Also you are going to want to pound on that partion to get any bad sectors
remapped before adding the partition back (assuming that a disk error is
what caused your problem).