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#61
Quite likely it saw the Data drive available space, and made the choice to put it's temp files there. I believe it's when it's short on space, after the Compatibility phase up front, and nothing is found to use, it will prompt you for a USB or another device to use for temp files.
I've had it use one of my data drives as a work space even when there's plenty of C: space. Spread the load around, runs more efficiently.