I don't think reset everything will erase the hard drive. The hard drive does not actually contain the operating system, which is a custom version of Linux. That is stored in non-volatile memory on the main board. Activation is based on the Access card of the Hr44. There is a diagnostics menu where you can analyze the hard drive or reformat it. It is accessed by holding down two buttons simultaneously on the remote, but I can't remember which ones. I will try to find that tomorrow, if no one else can come up with it.
If you replace the hard drive or add an external esata drive, and it is not formatted for DIrecTV's file system, it will reformat it automatically at power up. Once that is done, the HR44 Genie will start up and run, with no further activation needed. Nothing on the hard drive is needed to run. I am not sure that recent models have an eSATA port, but I have substituted eSATA drives with an eSATA dock many times.
My personal recommendation for a replacement hard drive is a Western Digital Red drive. They are built for continuous recording in surveillance systems and DVRs. Even if you are just watching live TV, the buffer is recording all the time. But WD Red drives are more expensive than ordinary PC drives. WD Red drives will probably outlive theDIRECTV satellites at this point so it may not be worth it. Mine is 12 years old and going strong. Don't try using anything over 4GB. And contrary to many recommendations in the forums, I would never use a SSD. SSDs have an upper limit to write cycles.