Recover NTFS data from a ZFS pool that was exposed as an iSCSI target

The problem was that the new motherboard's BIOS created a host protected area (HPA) on some of the drives, a small section used by OEMs for system recovery purposes, usually located at the end of the harddrive.

ZFS maintains 4 labels with partition meta information and the HPA prevents ZFS from seeing the upper two.

Solution: Boot Linux, use hdparm to inspect and remove the HPA. Be very careful, this can easily destroy your data for good. Consult the article and the hdparm man page (parameter -N) for details.

The problem did not only occur with the new motherboard, I had a similar issue when connecting the drives to an SAS controller card. The solution is the same.

I almost cringe to do this, but if you're serious about recovering your data, and you don't have backups, you'll need a higher-end data recovery service.
Watch out for fly-by-nights shops that will be peddling/scamming in your area, and go with an actual top-tier data recovery service. (And be prepared to pay $$$$ for it.)

Here's a list of what to look at:

1. Your data.

   • These services are expensive, generally starting at over $1000 a drive, so you need to decide if your data's really worth the cost of recovering it.
     
     

2. Recovery company's history.

   • If they haven't been around for at least several years, and in the data recovery field, they're probably not worth considering.
   • Top-tier companies will have a portfolio of the more interesting or challenging recoveries they've done. Take a look at those, and make sure the guys you choose have enough experience with water-damaged hard drives that it's just another day in the office for them.
     
     

3. Recovery company's clientele

   • Top tier companies are generally contracted by law enforcement and law firms for cases where someone deliberately tried to destroy a hard drive, so if you find one that's worked with a lot of police departments or on a lot of legal cases, they're probably a good one. (Generally this becomes public record at trial too, allowing them to disclose their involvement.)
     
     

4. Recovery company's facilities

   • Not that you should expect a tour, but you're looking for clean rooms, labs and proprietary or custom tools they've developed to recover data from physically damaged media.

And I don't generally do this, but since the other answer has a link I'd be a little wary of, I'll throw out a recommendation of sorts. These guys are good, (expensive as hell, but good), and if you decide to go down this route you should look at them, if only for nothing more than to compare other data recovery services against.

EDIT: And as mentioned in the comments by @Grant, DO NOT take it to a lower-quality data recovery service first, because both the data recovery process itself and the passage of time will do [additional] irreversible damage to your drives. If you decide later to try someone better or more qualified, your earlier decision will have increased the cost and decreased the results.