NFHK CF-Express Type-B to M.2 NVMe 2230 M-Key Adapter CFE for Xbox Series X&S CH SN530 SSD PCIe4.0 Expansion Memory Card
I'm pretty sure that the mystery slot on the back of the Xbox Series X is a CFexpress memory card slot. If an ethernet port is about 11.5mm wide then using the
Modder uses CFexpress adapter to attach Xbox Series NVMe SSD as expandable storage - VideoCardz.com
External Solid State Drives For Xbox Series X/S, External Host Hard Drive Conversion Box M.2 Expansion Card Box 32G BandWidth Support PCIE 4.0 Protocol - Walmart.com
Your standard SSD might work as an Xbox storage expansion | Ars Technica
Xbox Series X 'mystery slot' expected to fit CFExpress cards - Hardware - News - HEXUS.net
CFexpress To SSD M.2 NVME For Xbox Series X/S XSX CFexpress To SN530 1TB Replace | eBay
Modder uses CFexpress adapter to attach Xbox Series NVMe SSD as expandable storage - VideoCardz.com