in the other Alabama
The backdoor:You're lucky they were willing to do the exchange.
You voided your warranty the moment you opened up the unit and the replacement of the drive is easily detectable by TiVo and likely some stats about the drive (e.g. model #, number of LBA blocks, drive serial number, etc.) are uploaded to TiVo as part of the logs. Such stats were visible in TiVo Series 1 logs when the backdoor code that let you access the logs worked.
Still works on my Roamio with TE4. I can't tell if its contents have changed since the Series 1 however.