Michael, you said you tried "cat > foo.txt", but can you please try to "touch foo.txt" as Bill originally requested ?
If that comment still gives you an I/O error, is there any chance you can try with a different SD card, or try creating a file on the SD card from your computer ?
I/O errors suggest some kind of corruption in the file system on the card, or possibly even the card itself being damaged.
Michael, you said you tried "cat > foo.txt", but can you please try to "touch foo.txt" as Bill originally requested ?
If that comment still gives you an I/O error, is there any chance you can try with a different SD card, or try creating a file on the SD card from your computer ?
I/O errors suggest some kind of corruption in the file system on the card, or possibly even the card itself being damaged.