I have an Odriod XU4 and I put the latest Stretch version on, but in the /etc/apt/preferences.d/ folder,
the backports file has 2 references to “release a=jessie-backports”.
This does not look right. Shouldn’t it be “release a=stretch-backports” ?
glenser
Many thanks for your report. Indeed those entries do not make sense. Actually you can remove the whole file which AFAIK was to workaround an FFmpeg issue mainly that should not be present with Odroids anymore. Otherwise we would have new reports as those entries have no effect anymore 
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I just removed the file from our preparation script, which contains the stretch-backports line already, but obviously was patched in before C2 image was created: https://github.com/MichaIng/DietPi/commit/9d1608b4ad3d8cdb1343e7f71e8401624ee792da
Most likely the originating issue is not present anymore, hence the file not required. To assure, you can remove it and run an “apt update && apt upgrade” to see if any additional updates are offered.