dd cannot write to an ext4 HDD using oflag=direct

I attempted to write a file to an ext4 voulume on an external USB hard disk drive: $ dd if=/dev/zero of=file bs=1M count=10 iflag=fullblock oflag=direct dd: failed to open 'file': Invalid argument Then I tried it without oflag=direct and it worked. Then I tried to write a file using the original command to an ext4 volume on an internal solid state drive and it worked. Why couldn't dd write to the HDD?