I've seen solutions where you must boot from an external operating system, bla blah. Too complicated and time consuming.
To fix this fast and easy, start formatting, wait for "unmounting disk" message to appear, then go to Terminal, find the process with the following command
ps ax | grep fsck_hfs
You'll get something like
[email protected]:~$ ps ax | grep fsck_hfs
3693 ?? U 1:32.61 /System/Library/Filesystems/hfs.fs/Contents/Resources/./fsck_hfs -y /dev/disk2s2
3984 s000 S+ 0:00.00 grep fsck_hfs