adding drive to raidz1

[Andrew Snow]

> 	    NAME        STATE     READ WRITE CKSUM
> 	    tank        ONLINE       0     0     0
> 	      raidz1    ONLINE       0     0     0
> 		ad4s3   ONLINE       0     0     0
> 		ad5s3   ONLINE       0     0     0
> 		ad6s1   ONLINE       0     0     0
> 	      ad7s1     ONLINE       0     0     0

Here you have created a non-redundant stripe with two vdev members:
1.  a 3-disk RAIDZ1 and
2.a single disk.

So you can't ever remove the ad7s1 without data loss.

If you haven't written anything to the pool since adding ad7s1, you can 
probably yank the disk out and ignore any errors but the error messages 
will never go away until you recreate the whole pool from scratch