Raspberry Pi 5 Gets a microSD Express Boost, But at What Cost?

For Raspberry Pi enthusiasts craving faster storage without bulky M.2 adapters, developer Will Whang’s RPI5-SDexpress-Hat offers a sleek solution. This compact HAT+ taps into the Pi 5’s PCIe interface to add a microSD Express card slot—delivering near-SSD performance in a familiar form factor. With the Nintendo Switch 2 poised to popularize microSD Express, this open-source project arrives at a critical juncture, even as pricing challenges remind us that cutting-edge tech often comes with a premium.

Engineering the Speed: Inside the HAT+

The board connects via a PCIe Gen3 x1 FFC cable to the Raspberry Pi 5, featuring a WCH CH32V003 RISC-V microcontroller that handles hotplugging and safe unmounting—a necessity for avoiding data corruption. As one community expert noted:

"It will not work without a proper SD handshake and CMD8 command response with PCIe flags... the host controller needs to enable and reset the PCIe side."

Additional touches include an eject button, power LEDs, and two Qwiic connectors for I²C peripherals, making it a versatile expansion. Power is drawn from the Pi’s GPIO header, with onboard regulators like the LP5907 and PAM2306 providing stable 1.8V/3.3V outputs.

Benchmark Revelations: Fast Reads, Modest Writes

Performance tests, run on a Raspberry Pi 5 configured for PCIe Gen3 x1, reveal why microSD Express is a game-changer—with caveats:

Category                  Test                      Result
HDParm                    Disk Read                 631.68 MB/sec
HDParm                    Cached Disk Read          699.12 MB/sec
DD                        Disk Write                169 MB/s
FIO                       4k random read            48761 IOPS (195047 KB/s)
FIO                       4k random write           26290 IOPS (105160 KB/s)
IOZone                    4k read                   22803 KB/s
IOZone                    4k write                  137502 KB/s
IOZone                    4k random read            31366 KB/s
IOZone                    4k random write           178701 KB/s

Read speeds soar past traditional microSD cards, hitting 630+ MB/s—ideal for bandwidth-heavy tasks like video streaming or database operations. However, sustained writes plateau around 169 MB/s, aligning with high-end Class A1/A2 cards. Random I/O performance shines, suggesting solid gains for applications involving small file operations.

The Cost Conundrum and Open-Source Future

Samsung 256GB microSD EX card

Despite the technical prowess, microSD Express cards remain prohibitively expensive. A 256GB Samsung card costs $60—over double the price of comparable M.2 SSDs. Whang acknowledges this limits the HAT’s practicality today, opting not to mass-produce it. Instead, he’s released the design under an MIT license on GitHub, with KiCad schematics and a BOM available. Firmware binaries are included, though source code is pending, likely adapted from Whang’s earlier CFexpress HAT project.

This move empowers the community to iterate, especially if Nintendo Switch 2 adoption drives card prices down. For now, developers might prefer M.2 or CFexpress Type B alternatives for cost-effective speed. Yet, as Raspberry Pi 6 rumors swirl, this HAT+ could foreshadow integrated support, making microSD Express a staple in future SBC storage arsenals.

Source: CNX Software (Jean-Luc Aufranc)