>
> I just created a new partition of PLAN9 type there (according to
>> https://9p.io/wiki/plan9/setting_up_Venti/index.html), saved and
>> rebooted...
>>
>> ...and system failed to boot:
>>
>> boot: can't connect to file server: '/boot/kfs' does not exists
>>
>
> It seems the boot process does not like two PLAN9 partitions on SD.
>

I was wrong.
The problem is in disk/fdisk which probably damaged the partition table
disabling subsequent reboot.

If I partitioned SD under Linux, the problem was over...

Pavel