Please set the status bar to indicate »Document Profile« and post a screenshot of both images open as non-tabbed windows.
have an asterisk (Layer x, RGB/8*) in the header,
The asterisk indicates unsaved changes.
I have just found the answer after fishing around the forums, sorry I didn't find a related topic on my initial search so created a new thread.
Even though the Color mode was set to RGB on both, when I went to Edit> Convert to Profile in the destination file for some reason CMYK was selected for destination space. I changed it back to sRGB and now I can drop layers between the files no problem.
And I learnt something in the process! Thanks.
Convert to Profile should not be relevant for moving Layers between files, issues arising there should be governed by the settings in Edit > Colour Settings.