'Hard error reading fsbn' in 5.1 but not 5.0?

Just now, my 120GB western digital, immediately upon the mounting of all
filesystems during startup, made a faint "click", then my screen filled with
errors of the form:

ad6: hard error reading fsbn -# of -#--# error=10

That's not an exact quote, of course, since the messages scrolled by so fast I
couldn't actually read parts of them.  Those # signs are in place of large
numbers (most were 12-18 digits).  This was in 5.1-R, fresh install, in the
first 15G of the disk, just after having booted into DOS to create some DOS
partitions after the FreeBSD slice.

I booted back into 5.0, and am presently writing across the entire disk with
this command:

fdisk -I /dev/ad6 && dd if=/dev/zero of=/dev/ad6s1 bs=1m

At more than 33 minutes into it at 24+MB/sec, there have been no such errors and
the portion of the disk that was being mounted has already been covered by
zeros.

After dd completes/dies, I'm going to try to reproduce the error, but I figured
I'd bring this up now in case I've tripped over something someone else already
knows about.  I did notice that when I used fdisk(8) to look at the partition
table, the slice table showed the new DOS slice starting before the end of the
FreeBSD slice.  Assuming that was real overlap, and not harmless output
screwiness as a result of using two very different fdisks, is it possible for
such corruption to cause hard read errors?