Minisforum has released the ESP4B-850, a PCIe 4.0 x4 expansion card that packs four M.2 NVMe slots, a 20 Gbit/s USB-C port, and an OCuLink connector onto a single bracket. The card is aimed at builders and upgraders stuck with motherboards that only offer one or two M.2 slots, and it ships with an adapter that lets the OCuLink port connect up to four SATA drives or an external PCIe 3.0 device.
Bandwidth allocation scales with how many slots are populated. With one or two SSDs installed, each gets a full PCIe 4.0 x4 link. Adding a third or fourth drive drops each connection to PCIe 4.0 x2, topping out at roughly 3.9 GB/s per drive. That is fast enough for most workloads, though it will bottleneck the fastest Gen 4 SSDs when all four bays are filled. An integrated fan keeps thermals in check during sustained transfers.
Teardowns of the card identify its onboard switch silicon as PROM21L.6, a revision of AMD's Promontory 21 chipset, the same chipset IP used on B850, X870, and X870E motherboards, here repurposed to fan the card's single upstream PCIe link out to its NVMe, OCuLink, and USB-C connections, according to a hands-on review posted by Unikoshardware. That same chipset has its own small footprint in the Linux kernel: in May 2026, developers posted a prom21-xhci driver through several rounds of review on the kernel mailing list, adding hwmon support that exposes the Promontory 21 xHCI controller's onboard temperature sensor as a standard temp1_input reading, work Phoronix reported was drafted with help from an AI coding agent. The patch only covers thermal monitoring for the chipset's USB controller rather than the card as a whole, but it is a sign of the same silicon inside the ESP4B-850 picking up incremental Linux kernel visibility.
The ESP4B-850 is available now from Minisforum's online store for $136 (€125), with shipping expected to begin in late October 2026.
