ERROR: Unable to write data to disk idad1

[Didier WIROTH]

How I solved the problem:
1) I booted the live-cd (5.2-release cd2) 
2) Launched the "CDROM/DVD Use the "live" filesystem CDROM/DVD"
3) Launched /stand/sysinstall from the shell
4) and created the required partition from there.

I rebooted in normal mode and formatted (with newfs) the partition and was
able to use mount and use them.

What did not work was disabling the geom protection.
Setting the sysctl variable:
kern.geom.debugflags=16
did not work for me.
Some error occurred, the message was (more or less) that my device, the
disk (where I wanted to add the partition, which also contained a swap
partition) was busy because of the swap.