Digital Media Converter Pro can convert your DVR-MS files to display on your Seagate media player in the correct aspect ratio.
Choose the WMV for Desktop Viewing conversion, select the 'Very High Quality' profile to get better video quality than the default selection. This profile will also keep the original video dimensions and aspect ratio. This should work well on your media player.
If you have the Seagate GoFlex or Seagate FreeAgent media players, I suggest converting to MPEG4 with H.264 compression. Make sure to keep the same output resolution as your input file and to keep the aspect ratio the same. This should give you very good quality output.