mdconfig), uid 0: exited on signal 11

i386 current as of an hour ago
new kernel already installed and booted single user to do installworld

Trying to mount root from ufs:/dev/twed0s1a
Enter full pathname of shell or RETURN for /bin/sh:
id: not found
grep: not found
:/> mount -a
pid 46 (mdconfig), uid 0: exited on signal 11 (core dumped)

< nothing for three minutes, so whacked it >

^C:/> df
Filesystem    1024-blocks     Used   Avail Capacity  Mounted on
/dev/twed0s1a      253678   239710   -6326   103%    /
devfs                   1        1       0   100%    /dev
/dev/twed0s1d     1012974   340210  591728    37%    /var
/dev/twed0s1e     1012974   487620  444318    52%    /var/spool
/dev/twed0s1f    20308398 11711512 6972216    63%    /usr
/dev/twed0s1g    45626188 37208960 4767134    89%    /usr/home
/dev/twed0s1h       63254      280   57914     0%    /root