I'm pretty sure I'm missing MMC support in the kernel, or don't know what the libary or module needed is to support it on tc.
Slitaz also doesn't detect it, but in order to see SD cards on that distro, you have to install and load an mmc kernel module and put it into your rc.s startup.
I *think* that is what's missing from TC, either baked in mmc kernel support, or a kernel module for mmc that can be loaded at startup?
I'm thinking down the road here - had I put in an eMMC drive inside, I might have run up against the same problem.
I'll do more digging / learning, but if anyone has run across this before, let me know!