said by andyross :

With iGuide, if both the SD and HD have the exact same channel name, and you set a series recording, it will default to the lower channel number, which is typically the SD version.

If that is the problem, COMPLAIN to your local area to have them rename the channels (By me, Discovery is DSC, and Discovery HD is DSCHD.)

Otherwise, the only workaround is to delete the series recording and record by selecting from the guide, or even using a VCR-style manual recording.