Hard Drive Error Checking

I am using a Windows 7 Professional 64 bit PC with two internal hard drives. Drive C contains only the Windows 7 OS. Drive D contains all of my data.

I would like to do error checking on drive D. From Computer/Windows Explorer I right clicked on drive D and clicked Properties, Tools, Error Checking, Check Now. The message displayed was "Windows can't check the disk while it is in use - Do you want to dismount this volume first? Note: All opened handles to this volume will become invalid. Buttons are Force a dismount and Cancel"

When error checking drive C the dismount message does not appear.

Should I click the button "Force a dismount"? What does "All opened handles to this volume will become invalid." Will something bad happen?

Thanks very much for your help.

Bill