If you physically power down a hard drive by pulling its power plug,
and it happens to have been writing at th etime, you have a very good
chance of destroying the disk.
This happened to me a year or two ago... remember when I posted about
having to restore crater from backups? A power failure and no UPS
software caused my systems to do an uncontrolled power down and I wound
up with multiple disk errors and lost several disks.