Ok this might sound to be a silly task, but when you need to perform it, you might get frustrated. Actually several times we had to move or reassign disks to a different node/filer in our IBM Nseries Cluster. If you are not aware of the IBM Nseries then all I need to tell you its a Netapp box oemed as IBM. So again this post apply to N-Series as well Netapp clustered filers.
You might want to reassign a disk to a different N-Series Controller or Netapp filler what ever your vendor want to call it as long its running a data Ontap then the procedure is the same. When I first thought of this, I thought it will be something like disk unassign on the filler to remove the disk from. Then assign it on the other filer. Though it seems no command disk unassign is there. Trying all around the place then calling the IBM support we figured out the following is what need to be done.
On the filer or the gateway to remove the disk or the LUNs from run the below command:
disk remove -w <Disk Name>
Then on the filer or the gateway to add the disk or the LUN to run the below command:
Disk Assign <disk name>
Ah I hope this help some one avoid having too look for so long for how to do this tedius task 
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Please leave a comment if this work for you, as we like to hear your success stories or let us know if for some reason it did not work with the certain model or Ontap version you are running.
This didn’t work for me. In fact, it marked the disk as “failed” when I ran the “disk assign” part.
I recovered from that by going into Advanced mode (priv set advanced) and then using “disk unfail “.
From there, to unassign the disk, I used “disk remove_ownership “. Then I went back to normal mode with “priv set admin”.
Thanks for your feed back. It would be nice if you could supply your Data ontap version. As that command had perfectly worked for me on Data Ontap 7.2.3. We are upgrading soon to 7.2.6.1. I will update this post, when I get to try this command on new release.
To remove drive assignment in Data OnTap use:
To unassign:
disk assign 0b.30 0b.29 0b.28 0b.27 -s unowned -f
Run that for whatever disks you are trying to remove assignment for (do it from the head that currently owns the disk).
Many thanks N Lyne, that worked a treat for me on an IBM N6040, I had to also issue the command “options disk.auto_assign off” as they were re-aquired by the same controller before I got chance to assign them to he other one … whoops. Thanks again.