Funny, but I was having endless issues getting an SD card to work on an ethernet shield. After following every suggestion available on the internet, I finally broke down and bought a different shield. Got the Arduino Ethernet shield 2, and popped in the card. STILL didn't read. Argh... Ran across this thread, reformatted the card with the SD formatter, and it started working immediately. Thankfully, it still didn't work in the other shield, so at least the new shield was worth the purchase.
What makes me crazy is that there's this page: SD - Arduino Reference
It says, "Windows : right click on your card’s directory and choose “Format” from the drop down. Make sure you choose FAT as the filesystem."
It's so outdated that it says nothing about FAT32, doesn't say anything about cluster size, and literally says to use the OS formatting. That's what I read long before I found this thread, and it misled me. Someone needs to change it.