Check disk errors doesn't work on mounted volumes

Hi,

if you go to properties under a disk drive -> tools and then click check now
it opens a windows to check disk errors.
Now, if you go to windows computer managment -> disk managment and remove
all letter assigments for a drive (be careful which drive you pick!, if you
don't know this just don't do it) and then mount a volume in an empty
directory in other drive.
Then (without having any letter assigments for a drive) go to disk
properties and try to check disk errors in the tools tab, it will not open
the check errors window :-(.

Does it happen to you too? I guess the disk error checking program ask for a
letter to work :-(.