IIRC the 622/722 has compatible drive models programmed in firmware. They do this to insure compatibility. A drive with insufficient spindle speed, cache, etc. might cause the system to crash if the software is written to expect a certain response time. This seems likely as these devices can record 3 HD channels and playback 2 HD recordings. That is a lot of I/O. They only way to get unlisted drives to work is to somehow fake the metadata that the firmware checks. I do not know of any easy way to do this.