Your PC is running fsck (filesystem check) which checks the integrity of
your drives in a manner very similar to chkdsk (except that it actually
works!). The system is set up to automatically run fsck every x times
the disk is mounted (I believe the default is every 30 mounts, but I
could be wrong) just to make sure that there is nothing wrong with
them. I would not advise turning this off - and don't know how to
anyway - as it is a valuable check on your system's health.

As to why pressing C does nothing, I'm at a loss. I occasionally do
this when I'm in a hurry and it's never failed to work for me