Bizarre. Now, it has showed up two times: I pulled the uSD from the nano (after countless attempts to format it from the web console, and even trying my hand at this gist:
https://gist.github.com/kbeflo/8c85c084e9c5ae86b3367716a463e793
(for the record - it didn't work for me either). I tried stepping through the commands, one at a time, by hand... and still no dice.
So, finally, I pull the card, put it in a reader and plug it into the Mac. Mac declares it 'unreadable', opens disk utility, and allows me to format it (I selected MS-DOS (FAT), assuming this would provide the greatest interoperability).
Returned the SD card to the Nano, powered it up, and... Voila! The SD card was there. I did a 'Save Fstab' in the console UI, and then proceded to download and install a handful (okay, perhaps two handfuls) of modules and installed them to the SD card successfully. Eventually powered down the device... and now, a few hours later - upon reboot... no /dev/sdb anymore??
I'm obviously doing something wrong - as this just can't be this hard!
Thanks for any assistance.