As @ekashpureff mentioned, there's some gotchas that can happen (e.g. making sure that the disk really is a spare on the partner...), I really can't recommend strongly enough that you contact support and ask them to walk through it.
That being said, at a highlevel, the process is to remove ownership of the disk on the current node then take ownership from the other node. The commands are going to be different if you're using 7-mode vs clustered. For cDOT, it would be something along the lines of "storage disk removeowner" followed by "storage disk assign".
Andrew
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