problem booting : BAD MAGIC NUMBER

I've got netbsd 3.0 running on my ancient thinkpad 600e.  Because this
model can't boot the install cd, I did a manual install to the
laptop's disk by putting it into an enclosure and using another system.

I ran fdisk, mbrlabel, installboot, copied in the sets, ran MAKEDEV,
edited /etc/fstab, put it back in the laptop.  The installation using
this method works, with one glitch.

When I boot the thing, I get messages like this:

Checking for botched superblock upgrades: done.
Starting file system checks:
CG 0: PASS5: BAD MAGIC NUMBER

/dev/rwd0a: UNEXPECTED INCONSISTENCY; RUN fsck_ffs MANUALLY.

Then I run fsck_ffs manually, exit single user mode, and the system
continues booting fine.

Any idea what's borked on my system and how to fix it?

-russ