Could not use the move tool error msg...

Hi,

I'm working in two .psd files where I'm trying to move one group into another file by using the move tool and dragging from one file into the next.

However, when I try to do this, I get the attached error message saying "Could not used the move tool because the file was not found."

I get a similar message when I try to duplicate the group from one file into another, which says "Could not complete the duplicate group command because the files was not found.

Any idea how this can be resolved? Thanks!