Confused about fsck

[Colin Law]

Every so often my system checks my disk on bootup.  I thought that
this was done using fsck.  It takes a couple of minutes to do the
check.

I have an external 250GB USB disk that I wish to check.  I plug in the
disk and check it is not mounted (it appears as /dev/sdb1) and run
sudo fsck /dev/sdb1
This returns immediately saying that the disk is clean, whereas I had
expected it to take a little while checking things, so it does not
seem to be doing the same things that the check on startup does on the
built in disk.  I can't see any options on fsck to specify what is to
be checked.  Can someone explain what is going on?

Colin