Or, maybe the easiest thing of all would be to just keep the 2TB plugged in as the "backup" OS drive, and just make the 4TB drive a pure data volume? Wipe it so there are no boot files at all and just ...
The error Unrecognized partition found, index 1 disk layout validation failed occurs while converting MBR to GPT. This article lists the reasons and fixes for this error.
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