Why fdisk can't open root disk with MBR for writing?

[Yuri]

On 09/14/2012 19:23, Warren Block wrote:
> Did you actually try gpart?  GEOM prevents writes to providers that 
> are in use, but gpart should handle it correctly if the problem is 
> just that fdisk doesn't understand GEOM.
>
> # gpart set -a active -i 1 ada0
>
> If that fails, the provider is in use, usually mounted.  The safety 
> can be disabled, but it's better to figure out what has it in use. 

Yes it is the hard drive with mounted /. But changing the active slice 
should be safe in any case.
gpart worked.
This safety feature is protecting people from themselves. Sounds very 
familiar.

Thanks!
Yuri