I am not convinced you need the internal. I am convinced it is better to leave the internal alone, regardless of whether you add an X drive or not.
The reason I say this is I ran a HR20 for about 15 months, and then got a overtemp error that could not be resolved. I disconnected the X drive to go to the internal, and that did not work.
When I popped the hood I discovered that I had disabled the internal by removing the SATA cable and power, apparently just after receiving it 15 months ago, and hooked up the X only at that time.
So I hooked up the internal, but that did not immediately fix anything. It did a drive scan on its own and found some 3580 errors (on the drive I never even used). It fixed these errors in probably about 36 hours, and then was OK. I hooked the X back up and it has been working ever since, which was a couple months ago.
So, much of that is somewhat unexplainable, but the fact remains I ran with the X connected and the internal disconnected, for a considerably long time.
My understanding is that there is a hybrid OS, some things are in flash and some are on the HDD, which is why you can essentially stuff a blank formatted drive in and be good to go. It seems there is a fixed BIOS-like startup order, meaning it looks for a drive externally and mounts that if available, and if not it moves to the internal. But it will not mount both drives, which is unfortunate.