Move operation missing when dragging files

I have a user who is experiencing an odd situation. She's not being offered the 'move' option when dragging files from a folder in the file list window to a subfolder of the same folder containing the file she want to move.

For example, if she tries to drag File.txt up onto Subfolder it doesn't allow her to move the file there. Only copy is available. However, she CAN actually do the move other ways (dragging on to the folder tree or another tab, for example).

c:\temp\Subfolder
c:\temp\File.txt

I'm looking for any suggestions as to what could cause this. I appreciate any help.

Thanks,

Jason

Has Preferences / File Displays / Mouse / Allow drag and drop into sub-folders (without a qualifier key held down) been turned off?