leyboard fails in single user mode

I booted my Mac Mini into single user mode to run memtest. i was using a standard wired Apple keyboard and while the command-S worked fine on boot, and the system came up in the expected command line prompt, at that point the kb became completely unresponsive.

I plugged in a second keyboard and it worked fine. But no combination of connecting or reconnecting the Apple kb would make it respond.

I should point out that this kb works fine in normal use.

Any ideas would be appreciated.

Thanks
Dave