I spoke over phone with a Charter techy, and he claims that he was taught how to add an external drive to the DCX 3501-M during his training! For some reason, this knowledge is taught to techs, but never passed-on to us customers. He said process is "a little tricky."
I'm disgusted with Charter for leaving customers locked into this particular DVR. (The tech told me they support many DVRs, different regions of country only work with DVRs from particular vendors.) For high definition video storage, the motorola hard drive is too small. They should upgrade models every couple years..
I've had some weird problems with my DCX 3501-M. It forgets the "user settings", in particular the allowable video modes. Oddly, if I connect the DVR directly to the TV via HDMI, it doesn't seem to lose configuration. However, when I connect DVR via HDMI through a soundbar (acting as a switcher) the DVR loses configuration in an odd and unpredictable fashion.
I don't know enough about HDMI to troubleshoot this problem, and Charter techs know even less than me.
I agree with previous poster's general impression that the motorola DCX 3501-M is flaky. It does a nice job of upconverting a variety of video formats to 1080P60, that's appreciated. But the software/firmware is awkward & buggy.
BTW, what's with having to shut the sucker off just to switch user parameters, for instance turn on/off closed captioning? Dumb.