Problems replacing failing drive in ZFS pool

[Garrett Moore]

I have an 8-drive ZFS array consisting of WD15EADS drives. One of my disks
has started to fail, so I got a replacement disk. I have replaced a disk
before by:

 zpool offline tank /dev/da5
shutting down, swapping from old disk to new disk
booting
 zpool replace tank /dev/da5

This worked fine.

This time the failing disk was da3, and I tried the same thing:
 zpool offline tank /dev/da3
zpool status showed da3 offline.
shut down, swapped old disk to new disk.

When I booted again, I got:
 Code:

        NAME        STATE     READ WRITE CKSUM
        tank        UNAVAIL      0     0     0  insufficient replicas
          raidz1    UNAVAIL      0     0     0  corrupted data
            da0     ONLINE       0     0     0
            da1     ONLINE       0     0     0
            da2     ONLINE       0     0     0
            da3     ONLINE       0     0     0
            da4     ONLINE       0     0     0
            da5     ONLINE       0     0     0
            da6     ONLINE       0     0     0
            da7     ONLINE       0     0     0

I switched back to the old disk and booted again and then I could access my
data again, and da3 still showed as offline. I tried 'zpool online tank
/dev/da3' and after a few seconds resilvering completed and all 8 drives are
back online again, but with the 'dying' disk as da3 still.

I tried shutting down WITHOUT first offlining /dev/da3, and swapping the
disks, and when I booted I again got 'insufficient replicas'.

Why am I getting this error, and how come it worked ok the last time I
replaced a disk? And more importantly, how do I switch to my new replacement
disk without losing data?

Thanks.