I am assuming this is a rental unit. Otherwise I'd want to have additional details on how a purchased 8300HD was exchanged for an 8642HD.
As with all rentals, they are refurbished units when you do an exchange. They never will take a new one off the shelf outside of the first batch that come to the stores when the 8642HD were released, and those were very limited in numbers. In fact, if you remember at first, many stores were not allowing 8300HD exchanges for 8642HD units as they were only for special circumstance exchanges until referbs started to roll in. I would note to you that there are recent reports of some 500GB rental units floating around at stores instead of 320GB units, so if you do wind up going back to the Rogers store, it may be worth calling ahead or calling around and finding a store that has one with that size drive. There is also a smaller 160GB unit and you probably want to try and avoid that.
One thing to be cognizant of is that many at DHC have found the 8642HD to have a more sensitive tuner than the 8300HD. Check your signal strength. Something that may had marginally been enough before for the second tuner (which from my experience often registers lower signal levels) may not be now and that may be the cause of your problems (since the second tuner is what is often used for recordings).
Otherwise, yep, keep going back to the store until you get a unit that you are satisfied with having a good working hard drive, but I would be surprised if that was the real problem since they are often tested before being put back out into circulation. The only other thing to do would be do to a hard drive mod as outlined on the site, but since this is a rental unit, you can run into warranty problems and such. Granted, going this route (that is, installing a new SATA drive from scratch) would also confirm if its the hard drive, signal, or in rare circumstances the unit causing problem.
CM4228, CM4220HD, Samsung LNT3242H, AccessHD DTA1080D, RCA DTA800, CM7775, Samsung DTB-H260F, Samsung LN40B750