Well, whaddya know, just when I said I had never gotten an assertion
failure from the NCR drive, I finally get one :) It's the same one:

sd0(ncr0:0:0): COMMAND FAILED (4 28) @f86b7200
assertion "cp" failed: file "../../../../dev/pci/ncr.c", line 5494

I had gone off to take a shower, and when I came back, I switched out
of the VT that was running the X server, and that's about when I got
the assertion failure.
-- 
Name: Dave Huang     |   Mammal, mammal / their names are called /
INet: khym@bga.com   |   they raise a paw / the bat, the cat /
FurryMUCK: Dahan     |   dolphin and dog / koala bear and hog -- TMBG
Dahan: Hani G Y+C 19 Y++ L+++ W- C++ T++ A+ E+ S++ V++ F- Q+++ P+ B+ PA+ PL++