sata gives: taskqueue timeout, followed by reboot

[Dieter]

FreeBSD 6.2 running on AMD64
ad6 is sata disk connected to nforce4-ultra

Moving a filesystem via dump|restore pipeline,
source is ad6 (mounted read-only), dest is a sata-via-usb disk.
ad6 also has root and var, so there could have been other
disk activity, but dump would have been the lion's share
of i/o.  After grinding away for a few hours, I get:

kernel: ad6: WARNING - SETFEATURES SET TRANSFER MODE taskqueue timeout - completing request directly

1 hour 4 minutes later it rebooted, and did not cleanly unmount
the filesystems. (any of them, not just the ones on ad6)
No panic message found anywhere.

I haven't seen this message before, this disk has been running
fine for about 7 months.

What exactly does this warning message mean?

Shouldn't I have gotten a panic message?

Assuming that the kernel decided to reboot without unmounting
the ad6 filesystems, why didn't it at least sync/unmount the
filesystems on other disks?  Did it decide to not trust the
controller, or perhaps itself?