No idea. I'll try to have another go at some point.
I'm not sure how the SD card could be full, given that internal storage is 16GB. I don't think I have enough installed to fill it. At the bottom left in the menu (gmenu2x) it says '13.7/14.0 GB'. I take that to mean it's mostly empty, but maybe I misunderstand.
In any case, it seemed odd that it wasn't a problem with other .opk, files, etc. When I figured out how to make a proper OPK with the .love file, that OPK was copied fine, but the .love file alone hadn't been earlier. Since files should all transfer the same, I assume there's something up with the program, but maybe whatever code (on the GCW0) writes to the card can be overloaded with data. Of course that's all speculation, all I know is that it didn't work.