FatFs is a popular file system for embedded applications.
Occasionally though I was having trouble accessing some SD cards, a problem that's been reported here.and here.
I've looked at a working SD card and a not working one.
They were different models and size but the problem came down to the second card not having its logical partition defined as primary. Using Minitool Partition Wizard you can fix this by doing:
1. Right Click on First Partition
2. Select Set Partition as Primary
2. Click on Apply Changes
The partition goes from Logical to Primary.
FAT32 also works, I've tried up to 8GB.