A tech enthusiast successfully built a custom reader for the Toshiba MK4001MTD, the world's smallest HDD. By reverse-engineering a Nokia N91, the project decoded the SDIO-based ATA protocol to access data from this 20-year-old hardware.

Building a Reader for the World’s Smallest Hard Drive — MK4001MTD

Introduction

Back when flash storage was still very expensive, miniature hard drives offered a much better cost-per-gigabyte ratio. As IBM and later Hitachi produced the well-known 1-inch CompactFlash-form-factor microdrives, followed by Seagate, Western Digital, and GS Magicstor.

Rather than competing directly in that same 1-inch space, Toshiba introduced something even smaller: in January 2004, it announced a 0.85-inch hard disk drive, the MK4001MTD. This drive later appeared in products such as the Nokia N91, where it provided 4 GB of storage in a feature phone. Toshiba’s 0.85-inch HDD was also recognized by Guinness World Records in 2004 as the world’s smallest hard disk drive. Not long afterward, flash memory rapidly became cheaper, larger, and more practical, and drives like this disappeared from the market.

History and Initial Failures

Researching how to read the MK4001MTD revealed that no existing projects had successfully managed to read data from the drive. The pad layout resembles an MMC-style connector, but with more pads than a standard SD card, similar to MMCplus which supports an 8-bit data bus.

Initial attempts to connect the drive to a microSD card reader failed, even with external power. A custom reader built around the USB2240 to support 8-bit MMC also failed to elicit a response. The drive did not respond to standard commands like ACMD41, CMD55, or CMD8, proving it was not behaving like a normal SD or MMC storage device.

Reverse Engineering

To understand the protocol, a second-hand Nokia N91 was used as a host device. Wires were soldered to the drive contacts to probe signals with a logic analyzer while the phone was operating.

The traces confirmed this is a 4-bit SDIO interface rather than a normal memory-card mode. The bus activity showed that ATA-style commands, such as IDENTIFY DEVICE, were being transported over the SDIO interface.

Using OpenClaw and reasoning-focused LLMs, the project successfully reverse-engineered the logic traces. A Raspberry Pi Pico (RP2040) was used to run firmware that exposes the drive as USB mass storage, translating USB storage requests into ATA commands tunneled over SDIO.

Hardware Design and Results

A custom PCB was designed in KiCad, featuring an RP2040 paired with spring-loaded contacts to mate directly with the drive.

The SDIO clock is currently set to 10 MHz. While slow by modern standards, it is functional and usable for accessing the data on these vintage drives. The project successfully demonstrated that Toshiba carried standard ATA commands over SDIO without excessive proprietary obfuscation.