SanDisk Extreme SSD 1TB

I have tried everything I can think of, but my brand new SanDisk Extreme SSD 1TB will not mount to my (unrooted, unmodified) Galaxy S8.

I formatted it into exFAT. I tried that on Mac & a PC. My Chromebook will mount it, my Mac and PC desktops will mount it. My S8 will not.

When I plug it in (USB C to USB C cable as provided - no OTG adaptor used and should not be needed but even that fails to help as I tried one) I get the “Use USB to” pop-up and I select FILE TRANSFER and no matter what, it always instantly reverts back to CHARGE CONNECTED DEVICE. It will only try to charge, not stay as a “file transfer device”.