(ZFS) zpool replace weirdness

I've been playing around with zfs for a bit, and ran into a problem
where i corrupted an entire drive (on purpose) by way of dd
if=/dev/urandom of=/dev/ad12 .. as expected, the zpool noticed:

su-2.05b# zpool status
  pool: array1
 state: ONLINE
status: One or more devices could not be used because the label is missing or
        invalid.  Sufficient replicas exist for the pool to continue
        functioning in a degraded state.
action: Replace the device using 'zpool replace'.
   see: http://www.sun.com/msg/ZFS-8000-4J
 scrub: resilver completed with 0 errors on Mon Jul 23 23:05:53 2007
config:

        NAME        STATE     READ WRITE CKSUM
        array1      ONLINE       0     0     0
          raidz1    ONLINE       0     0     0
            ad10    ONLINE       0     0     0
            ad12    UNAVAIL      0     0     0  corrupted data
            ad14    ONLINE       0     0     0
            ad16    ONLINE       0     0     0
          raidz1    ONLINE       0     0     0
            ad18    ONLINE       0     0     0
            ad20    ONLINE       0     0     0
            ad22    ONLINE       0     0     0
            ad24    ONLINE       0     0     0

errors: No known data errors


now i want to resilver that disk, but the problem is this:

su-2.05b# zpool replace -f array1 ad12
invalid vdev specification
the following errors must be manually repaired:
ad12 is in use (r1w1e1)

but nothing is using that disk as far as i can tell! has anyone
successfully done this?

would it be better to use slices instead of whole disks for zfs on
freebsd? i want to get some experience with this so that i know
what not to do when a disk breaks for real :)

-- 
Best regards,
 Sverre S                           mailto:ss.alert_at_online.no