I thought I could post photos!

"A source volume could not be locked as it is in use by another process. Do you wish to attempt to force a dismount on the volume?
ALL OPEN HANDLES TO THIS VOLUME WOLD THEN BE INVALID."

Then 2 buttons: [Yes] or [No]. If I select Yes, then another message comes up and says:

"A source volume could not be locked (640)." [OK]

Then it returns to where I started. What does it mean?

Thanks!