I finished responding to a thread at linuxquestions.org regarding a Slackware user who was unable to mount a 6-in-1 USB card reader. Unfortunately, my final advice was to either recompile the Slackware kernel to enable CONFIG_MULTI_LUN or to install Knoppix.

In the course of googling for my post, I found a good explanation of why the CONFIG_MULTI_LUN option is disabled by default: "the LUN stuff is a hack at best." For single-slot CompactFlash devices, the LUN code will detect additional phantom devices.