my 256GB SSD drive in MacPro 2013 started to fill up recently.
i went on short googling to see how to extend it without relying on NAS of course. and i get nice offer.
it seems that good people in the internet found a way to interface typical NVMe disk drives with the socket Apple uses. and so i became user of 1TB Samsung SSD drive. that upgrade gave me also speed bump - on encrypted drive transfers shoot up from 500MBbs (reading) and 380MBbs (writing) to 1.3GBps for both reading and writing (disk itself can do 2.5Gbps but PCIe bus in MacPro is limiting factor here).