I'm pretty sure you will get a bunch of different answers for your question and I hope I learn something new from what somebody else has to say …
In my situation , w my lap top here's what I do ….
option # 1 , I use an external HD that spins at 7200 …
the only minor draw back I have to that option is my lap top only has a few USB ports and I like to hook up my 2i4 to it's own USB port …once I add a midi keyboard I end up having to put that on a hub…
When I'm working up something from the ground up from scratch I'm perfectly OK starting out using my internal HD for starting up my projects and experimenting around .
My lap tops internal HD spins at 5400 and I use it for convenience primarily ….
This HD was setup with a partition for the OS + programs and a partition for Data from the laptops manufacturer ..
once something I'm working on takes shape I will save it to the Data partition only because I don't want it to eat up the space on the OS partition …
I may also decide to send it over to the external HD so I also have a copy of my parent project while it is still very young …
In this phase , I'm pretty much talking about song ideas that aren't hitting my CPU all that hard to begin with …
While my scratch projects are still in Limbo , they stay where I can get to them fast and easy with out having to hook up and route a bunch of external gear and HD's …
The reason for this approach is I like to be able to take my lap top out of it's cary bag in a moments notice a get busy as fast as I can once an idea hits me …
On true proper projects that are more developed and when I know that I will be working on a specific song or Idea
with the intention of putting my best foot forward .
I will run an external HD for streaming the project and another external HD that has samples and loops on it …
Just so you know , I'm running a middle of the road lap top as it came from the factory w the exception of de junking it as best as I could ….also I do not have mega libraries or super synths to run over here ….
I have no idea if I can pull out the optical drive and put another drive there or if the mo bo even supports that ….
Ideally yeah if I ever decide to have a laptop that is custom made I would probably be OK w that as an option .
Even this little chugger I have can pull down 24 to 35 tracks w out breaking a sweat before I need to stream from another hard drive …as I've said, I think it is OK to use the internal drive for scratch ideas and then relocate them to stream from and External HD
I'm not a power computer guy so I do hope someone else chimes in that is one and gives you better answers …
have a good one ,
Kenny