Dig out an old USB 2.0 drive between . Avoid USB 3.0 drives—they are backwards compatible, but their higher power draw and faster negotiation timing sometimes confuse the PS2.
In specific builds of uLaunchELF, the USB driver modules are loaded at application startup. ulaunchelf mass empty
Diagnosing and Resolving USB Mass Storage Device Recognition Failures in uLaunchELF Dig out an old USB 2
No installer under mass folder (I'm probably doing ... - GitHub ulaunchelf mass empty