That's not what happens on Rogers. I doubt it happens on Videotron either, so the CSR is likely incorrect - he is certainly incorrect about the drives acting as one.
I'm sure nothing is done to the internal drive when you add an external drive. Also, the drives are not "recognized as one", there is a page in the diagnostics which shows the two drives as separate, with different sizes, model numbers, etc. If you had an external drive, it went bad and had to replace it, this would not remove the programming from the internal drive - makes no sense, so I have no faith in what the CSR told you. Here's the link to the diagnostics. One of the later pages shows the two drives and you can check for yourself.
I would say that the information you got from Videotron is incorrect. Some people in the Rogers thread have been able to swap back and forth between external drives without issue and certainly, nothing happens to the internal drive, unless there is some sort of problem.
An easy test would be to unplug the external drive (using the appropriate procedure as outlined in the External Drive thread) and try to access the information on the internal drive (programmes that are recorded there). You can then plug the external drive back in (using the appropriate procedure) and the programming will still be there for both drives) If you are concerned about losing the programming on the external drive, copy it first.
As I have stated in the Rogers External Drive thread, I don't believe there is any "formatting" that actually takes place, since the drive is recognized almost immediately and the usual "formatting sounds" that you would hear when you format a drive on a computer are absent. I believe that the drive is simply "recognized", which is why you can plug in a "previous" external drive and access the programming on it and then plug in another external drive and access programming on it. Also, the internal drive is certainly not formatted when and external drive is added, otherwise you would have lost any information on the internal when you originally added your external drive.
I cannot guarantee any of this, but I am relating information gleaned from the Rogers External Drive thread and my own experience from unplugging the external drive and plugging it back in - which happens to people all the time. Very few have done the external drive "swap" (using several external drives), however, those that have, have reported success. Good Luck.