#mkfs -t vfat /dev/sda1 mkfs.vfat 2.10 (22 Sep 2003)Device mounted using subfs, please do not access it while creating filesystem
A quick google shows that subfs/submount is an attempt to manage removable media. I guess your distro is trying to auto mount the filesystem for you. The command: "submount -u /dev/sda1" may be enough to umount the drive so that you can format it.
-- sg