Thank you for your question and reaching out. I can understand you are having query\issues related to Disk mirroring in Windows.
It's crucial that the disk you're adding has no volumes and should be the same size as or larger than the source disc. Deleting volume is therefore necessary. The "Mirror" option on the source disc will stop being greyed out as soon as Disk Management locates an unassigned disc that is appropriately large.
Make sure they are GPT if its more than 2 TB in size.
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