fsck and dump freeze freebsd. any ideas?

[Edward Sanford Sutton, III]

  The smartctl tests on the three drives came back saying they were checked 
100% without error; bad sectors have always caused an error and an aborted 
scan at the point of trouble for me in the past.
  I 'thought' I recalled seeing a panic along the lines of ffs_free or 
something related to free blocks or something at one point; maybe that was an 
error with fsck on the live system when manually ran. I will try to 
reproduce. Otherwise I do not know that fsck crashes; the entire system 
locks. I will see if I can reproduce the problem as a panic.
  There were some errors fsck seems to think it cleaned. A couple still exist 
indicating wrong counts about '12 should be 4' or '4 should be 0' type stuff. 
One error had a ridiculously large seven digit or so number that fsck said 
should be reduced. I had made a many terabyte file with a dd write to the end 
of a file of the largest size it would let me create and have since deleted 
the file; maybe that was what the reference was to. After fixing some errors, 
the fsck still causes a freeze and at what seems to be about the same point.
  The filesystem was unmounted for the dump and fsck has been attempted both 
mounted and unmounted. The filesystem has (ufs, NFS exported, local, 
soft-updates) reported for features by mount under normal operation.
  The freeze with dump did occur during a snapshot creation, which I found I 
cannot kill with a -9 (kill attempted minutes before the crash which still 
would not stop the creation).
  The powerdown question is more for how to handle an unsafe powerdown/crash 
on an active system. If I need to read from a partition, would read only 
leave it completely clean? Is there a way to operate on a file system which 
is treated as more of a ramdisk of changes and keeps the real partition 
unmodified (giving results like Faronics deepfreeze software or qemu disks in 
snapshot mode)?
  Would a zfs mirror configuration handle the unexpected crash/powerdown? 
Would it just report and fix the corruption, mention what files/structures 
weror impacted, offer restore of that data from a recent snapshot, or just 
say it is time to restore from a backup?
  Thanks again for the feedback,
Ed Sutton