disk is AWOL

[Dieter BSD]

8.2 amd64
ad8 is a 3TB Seagate on nforce4-ultra controller

At boot:
ad8: 2861588MB <ST3000DM001 9YN166 CC4B> at ata4-master UDMA100 SATA 3Gb/s
DEBUG g_part_gpt.c gpt_read_hdr() ad8 succeeded with pp->sectorsize=512

An hour later:
# dd if=/dev/ad8 bs=4k count=1 of=/dev/null
dd: /dev/ad8: No such file or directory

BUT: there was no "ad8: FAILURE - device detached" or similar message
on the console, in dmesg, or in /var/log/*.  The disk just disappeared
without a peep from the kernel. What's going on?