If your Comcast box has a composite output, you'll be able to record SD shows. Use RCA composite video and audio cables between the box and the recorder. You'll have to set the box to the channel you want to record and leave it on. Then set the HDD/DVD unit to record from video input (L1 rear, L2 front) for the appropriate length of time. The manual explains this. The manual is available from Funai who built the unit with the Magnavox name: http://funaiport.com/upload/MAGNAVOX/Manual/MDR537H_EN_OM.pdf
. See pages 19 and 42. If your TV and/or cable box only have a limited number of video inputs/outputs, you may need to daisy chain the composite video cables from the cable box to the DVD/HDD unit's video in. Then connect another composite cable from the DVD/HDD video out to the TV.
This Magnavox unit can only read analog (NTSC) cable video signals without a set top box. You will not be able to record a show and then watch another with the cable box.
I hope this helps.
(I know that my cable system's box has a composite video out. So it does work with most of the Comcast boxes. Many of the manuals are available here: http://customer.comcast.com/help-and-support/cable-tv/cable-box-user-manuals/ )