sysinstall: label editor:unable to write data

[Per olof Ljungmark]

I am trying to modify the da0s1 slices on a harddisk under 5.4 using 
/usr/sbin/sysinstall.
I delete an existing slice and create two new ones in the old freed 
space. When writing the changes I get:

ERROR: Unable to write data to disk da0!

There are no naming conflicts
Single user and multiuser no difference
There are only two existing slices mounted, / and /tmp no separate 
slices for /var /usr and so on.
What am I missing here?
Thanks,