I don't even know where that dialog box is. Your version of Lightroom looks way different to mine, is yours the latest version?
Ok, an update on yesterday's disappearing Raw file issue. I tried to import and convert today's shoot and all seems to have gone according to plan. The converted DNG files are there in a new folder, and the Raw files are still in the original folder. Whatever I did wrong last time, I seem to have omitted or corrected (accidentally) in today's import.
I'm using Win 7 with a Classic interface. So even though my LR looks different I do have the newest version installed.
How are you converting to DNG? Are you doing it at time of import with the option in the import dialog to "Copy as DNG"?
That dialog box I posted a screen shot of is from converting to DNG after the files are imported. The option is found in the Library menu in the Library module.
But I tested the "Copy as DNG" option in the import dialog after I had selected the option to Delete original RAW files in the dialog box I posted and the original RAW files did not get deleted.
Take a look in your "Working" folder for the original RAW image files. They may of been copied over before they got converted to DNG.
If they aren't there then search you external drive for one of the File Names to see if they are in some other folder than you expected them to be in.