Can you be more specific as to why iTunes won't play them? The DRM should not be an issue provided you are still using iTunes on the same PC. Using a different drive to locate your iTunes Music Folder should not matter.
You are aware that simply copying files (MP3 or AAC or whatever) into your iTunes Music folder is not enough for iTunes to recognise them - you need to choose Add Folder to Library or Add Files to Library within iTunes.
I recommend turnnig on "Keep iTunes Music Folder Organised" and "Copy Files to iTunes Music Folder When Adding Files to Library" in iTunes (in Edit -> Preferences -> Advanced). Then if you are importing files, copy them to a temp folder (on your C: drive perhaps), choose Add Folder to Library, which will copy the files (to your external hard drive in your case) in and reorganise if necessary (i.e. order by Artist then Album). You can then delete the temporary folder if you wish.