Why does mkfs fail with 'Read-only file system'?

Question

I'm trying to format a pen drive using mkfs, but it fails with: Read-only file system. Here is what I have tried so far:

$ sudo hdparm -r0 /dev/sdb1

/dev/sdb1:
 setting readonly to 0 (off)
 readonly      =  0 (off)
$ umount /dev/sdb1
$ sudo chmod 777 /dev/sdb1
$ sudo mkfs -t vfat /dev/sdb1
mkfs.vfat 3.0.13 (30 Jun 2012)
mkfs.vfat: unable to open /dev/sdb1: Read-only file system

What should I do?