how to fix "CORRUPT" partition table in mirrored drive

[Warren Block]

On Thu, 25 Jun 2015, William Dudley wrote:

> I have a mirrored drive:
>
> gpart show -l
> =>      63  78156225  ad0  MBR  (37G)
>        63  78156162    1  (null)  [active]  (37G)
>  78156225        63       - free -  (31k)
>
> =>       0  78156162  ad0s1  BSD  (37G)
>         0  41943040      1  (null)  (20G)
>  41943040   2097152      2  (null)  (1.0G)
>  44040192  34115970      4  (null)  (16G)
>
> =>        63  3907029104  mirror/gm0  MBR  (1.8T) [CORRUPT]
>          63  3907029105           1  (null)  [active]  (1.8T)
>
> =>         0  3907029105  mirror/gm0s1  BSD  (1.8T)
>           0   335544320             4  (null)  (160G)
>   335544320   209715200             5  (null)  (100G)
>   545259520   209715200             6  (null)  (100G)
>   754974720  3152054385             7  (null)  (1.5T)
>
> I tried to fix the corrupt partition table like this:
>
> gpart recover mirror/gm0
> gpart: recovering 'mirror/gm0' failed: Function not implemented
>
> So how do I fix the corrupt partition table?  Can I un-mirror the two drives
> and then do the gpart recover and then re-mirror them?

gpart recover only works with some partitioning schemes.  GPT has a 
backup copy of the partition table that can be used to recover the 
primary one.  MBR and BSD partitioning schemes do not have redundant 
metadata for recover to use.