What backup software are you using? I have a 3c and got fed up with the Psiwin program. It would only convert to rTF for the 3a, with the 5 series, I seem to remember the drag and drop process you describe.
It could be that as you drag, the software is trying to convert the file on the pC into the 5 series equivilent. try changing the preferences to a format (maybe RTF) that the series 3c will work with.