I was on a v15. Went straight to 18g.
- My card was FAT16, if that matters. You could also try resetting all configuration to defaults. I think there's a menu for that.
- It shouldn't matter on a FAT card, but is the firmware folder name all lower case?
- When you copied it, is there any chance that there are some attributes set that could interfere with the system seeing it?
- Have you tried redownloading? It may have been corrupted.
- Have you tried making the firmware folder by hand, then copying the BIN file into it?
- Consider formatting the card again, if you haven't already, or try another card if you have one. I've had a few cards that were intermittently weird with one device, but not another.
Just some thoughts.