gmirror crash writing to disk? Or is it su+j crash?

[Zaphod Beeblebrox]

So I have a system running:

FreeBSD walk.dclg.ca 9.2-RC3 FreeBSD 9.2-RC3 # r254952: Wed Aug 28 03:02:55
EDT 2013     root at walk.dclg.ca:/usr/obj/usr/src/sys/STRIKE  i386

and it has two 2T SATA disks.  To keep this post short, the crash.txt is
here.

https://uk.eicat.ca/owncloud/public.php?service=files&t=fea9d25579fe0c4afb808859e80e1493

now curiously, while running a "make -j4 buildkernel" ... almost every time
... it crashes with:

g_vfs_done():mirror/walke[WRITE(offset=516764794880, length=65536)]error =
11
/usr: got error 11 while accessing filesystem
panic: softdep_deallocate_dependencies: unrecovered I/O error

... no error report from the hard drives, simply an error report from the
mirror.

The filesystem is ufs with su+j... but I'm not sure this matters here.