SD cards can be tricky to format in FAT32....check this out
SD Memory Card Formatter | SD Association
If all else fails you might want to try an SD Card Formatter like these good folks supply
I had a Sandisk extreme Pro 128 GB card that wouldn't do the previous update so I reformatted it using the SD formatter and the newest update went just fine
I run several raspberry pi servers that use sd cards and find they need to be properly formatted using SD Card Formatter to work properly in Linux
I pretty sure the updates are written to the sd card first via your smartphone and only once the download is complete does the microcontroller on the
Nano+ read the sd card to update it's firmware.