how to fix "CORRUPT" partition table in mirrored drive

[William Dudley]

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?

A google search of the above error message turns up nothing at all
relevant.

Thanks,
Bill Dudley