There are no built-in drive BAYS in the Mac Pro 7,1.
There is a space above the I/O card, where you can bolt in the device I pictured above, or similar devices made by other vendors. Very close by, but NOT lined up to just plug in a drive, are two SATA data CABLE ports, a splittable power port, and a USB-3 port. Take a look at that picture again, because that IS your internal drive solution.
once you install that holder and cables:
"Fusion Flex J3i provide support for mounting two 3.5-inch SATA hard drives of your choice, AND
the Flex’s custom design provides mounting support for an additional 2.5-inch SATA SSD."