Kiko,
I had something very similar.
I have a 8.2.0-release-p1-x64 FreeNAS server that has been running for 2 years.
I have 2 x 2TB drives in a mirror configuration.
It recently notified me that there were some data errors in about 5 files, although I could read the files and they didn't seem corrupted so I think the mirror was doing its job. (The pool was not degraded at this time, and everything was "ONLINE".)
I manually ran a scrub to clear out the errors from the command shell. The drives spun up and I heard activity for about 2 seconds and then suddenly the system crashed with a Fatal Trap 12.
I rebooted the system and during the bootup sequence I could see that it mentioned that now the pool was degraded and something about the geom/gpt was corrupted on one of the disks in the mirror. When the system tried to import the pool it crashed with another Fatal Trap 12.
I thought it might be my server dying, so I pulled the disks and put them into my backup server; and loaded FreeNAS 8.2.0-release-p1 onto it.
When I set it up and tried to import the volume the new machine crashed with Fatal Trap 12.
I was now extremely concerned that I had lost my data, despite having a mirror configuration. (Due to a set of circumstances that I won't get into here my backup was already in bad shape; in fact this error happened on my main server while trying to recreate my backup that had its own problem... so I was really in a pickle!)
There are a number of "recovery" type options on the zpool import command that have been mentioned in a lot of posts, but I wanted to secure whatever I had of my data before experimenting-- I figured most of the actual data was there, just somehow the headers or something were destroyed. I wanted to duplicate my drives so I could work on them without fear of destroying whatever data I had.
First I ran memtest86+ on my backup server and it made multiple passes with no errors.
So I downloaded FreeBSD 8.3 and installed it to get a FreeBSD machine working on my backup server that has more tools/flexibility than the FreeNAS system does. (I chose 8.3 because it was close to the 8.2 that my FreeNAS box was running, but 8.3 added an option to import a pool read only which is something i wanted while trying to recover my drives that wouldn't import)
FreeBSD 8.3 was able to see my mirrored drives, and had the same status the the pool was degraded and one drive corrupted somehow. I did not import the pool though, since that was what had crashed both my main server and my backup server.
I installed ddrescue and copied my mirrored drives onto fresh drives. They copied with zero errors, so I don't think the original drives were having any hardware issues.
Once I had the copies I put the originals of my mirror in a safe place and installed just the copies of the mirror on the FreeBSD 8.3 machine. I was then able to import the pool (readonly) and it didn't crash. I replicated the filesystem onto yet another fresh disk (so now I have all my data on a non-mirrored disk) I checked and on that fresh disk I can import the pool and see the data.
Now I'm trying to figure out if the mirror can be repaired. I removed my backup disk so I only have the mirrored array on the FreeBSD machine, I booted it up, and I tried to import the mirrored pool (same as before), only this time I didn't use the -o readonly=on flag. When I imported without the readonly flag the machine crashed again!
I've tried 3 times in a row now, and trying to import the mirror causes a Panic crash.
If I disconnect the disks, reboot the system, and "export" the pool so that FreeBSD "forgets" about the pool; then shutdown, reconnect the mirrored disks, reboot, and then import the pool -o readonly=on there is no problem.
So something about the set of errors on the mirrored disks does not allow ZFS to import the pool normally (causes a crash even!); but it can import the pool "readonly"
I'm loosing a bit of faith in ZFS here if it seems to be able to read the pool "readonly", but when I try to import the pool normally the system crashes-- this isn't what a well behaved filesystem should do, no matter what the problem on the disk is, it shouldn't crash/Panic!
I guess I'll need to move up to FreeBSD 9 and see if the problems are fixed in that version.
I welcome any comments, questions, etc. from those more expert than I. I felt I should post this since I've learned so much from reading what everyone else does. Maybe this will strike a chord with someone or give them another avenue to try when their data appears lost.