CURRENT: Issue with ZFS and 2TB WD HDD (WDC WD20EURS-63Z9B1 80.00A80)

I'm running a newly setup FreeBSD 9.0-CURRENT/amd64 SMP box. The former 
OS was FreeBSD 8.1-PRE/amd64. Attached to the system are three WD 
harddrives with ZFS as filesystem on GPT partitioning scheme. Two 1TB 
HDD are spanning a volume with 2 TB capacity which is quite fast due to 
parallel access through ZFS. The underlying layout is, as mentioned, 
GPT, bot devices are named /dev/ada1p1 and /dev/ada2p1. The thir drive 
is a WD Caviar Green 2TB HDD acting as a compressed ZFS backup media, 
also based on a GPT partitioning scheme and known as device /dev/ada3p1.
In FreeBSD 8.2-PRE/amd64 I can use the JBOD volume resembled from two 
1TB HDD without problems, mounting is all right, dismounting, also with 
the 2TB HDD, labeld as BACKUP00 volume in ZFS. Doing a "zpool export" on 
both ZFS volumes works fine in FBSD 8, importing is also no problem. The 
system's log does not show any kind of irregularities, errors or 
something else indicating soemthing weird with one of the three devices.

Exporting both volumes in FreeBSD 8 works. But importing them in FreeBSD 
9.0-CURRENT/amd64 as with the most recent make world of today fails on 
the 2TB HDD (ZFS pool/volume BACKUP00). Issuing "zpool import BACKUP00" 
results in

cannot import 'BACKUP': no such pool available

and on console I receive message

ZFS WARNING: Unable to attach to ada1

But the drive is and was ada3!

Looking with zpool status, I get the follwoing picture:

   pool: BACKUP00
  state: FAULTED
status: One or more devices could not be used because the label is missing
         or invalid.  There are insufficient replicas for the pool to 
continue
         functioning.
action: Destroy and re-create the pool from a backup source.
    see: http://www.sun.com/msg/ZFS-8000-5E
  scrub: none requested
config:

         NAME        STATE     READ WRITE CKSUM
         BACKUP00    FAULTED      0     0     0  corrupted data
           ada3      UNAVAIL      0     0     0  corrupted data

   pool: THOR00
  state: ONLINE
  scrub: none requested
config:

         NAME        STATE     READ WRITE CKSUM
         THOR00      ONLINE       0     0     0
           ada1p1    ONLINE       0     0     0
           ada2p1    ONLINE       0     0     0

errors: No known data errors


This picture differs from what I see i FreeBSD 8.2-PRE/amd issuing the 
same command!

I did a full "zpool scrub" on both pools - no effect.

Surprisingly, the GPT partition of the pool BACKUP00 isn't shown in 
FreeBSD 9, while I see ada3p1 in FreeBSD 8.2.
gpart show ada3 lists this:

=>        34  3907029101  ada3  GPT  (1.8T)
           34        4062        - free -  (2.0M)
         4096  3907025039     1  freebsd-zfs  (1.8T)

The drive in question is a WesternDigital Caviar Green WD (<WDC 
WD20EURS-63Z9B1 80.00A80>     at scbus7 target 0 lun 0 (pass3,ada3)).

Can someone explain what#s going on? The drive in question is one of the 
new 4k sized physical block devices. Why is FreeBSD 8 dealing with the 
HDD, FreeBSD 9 not? Any help at this point? Hope the volume is not lost.

Thanks in advance,

Oliver