The card in question is a SanDisk Ultra 16G micro SD. It has a "10" with a circle around it. Initially it worked fine and showed a 16G capacity, but later it began showing only a 31M capacity.
Some searches indicate this is some kind of common failure mode. I am using it in a USB to SD card reader, so I don't get the whole story like I might if it was directly plugged in to some kind of linux system.
People have "recovered" from whatever this problem might be in various ways. One is to use some kind of formatting utility that runs under Windows. Nothing of the sort seems available for Linux. Another scheme is to plug the card into some kind of camera or device and have that device format the disk.
I am just going to label this drive and set it aside for the time. It would make a nice project perhaps to work up an SD interface driver for something like the Rockchip board, read the datasheets and get a handle on the SD command set and experiment with this card and reformatting it.
Another thought is that even 32M is adequate for what I would like to do with this card -- namely to put U-Boot on it. But I am struggling with various issues already with the process of installing U-Boot, and adding a flakey card to that project would be exceedingly unwise.
Tom's electronics pages / [email protected]