True alas BN, I was thinking about the last generation of MacBooks, laptops generally and big fat do-it-yourself desktop Intel/AMD mobos.
AFAIK there's no way of adding another SSD to the new MacBooks and I'm not even sure you can change the one that's factory fitted either.
Most new laptops fall pretty low on the iFixit scale these days - having been mostly assembled by machine in cleanrooms all they need is a little oriental wageslave to irretrievably hot-glue the last bits together.
The OS'es are getting a bit strict too - I can't upgrade my new'ish Mac Mini to Yosemite from Mavericks because it won't TRIM my added-on non-Apple Samsung SSD.
Buy it, use it and if anything goes wrong or a new OS version is incompatible just bin it and buy another.
So it goes...
Mac
I did love the days when you could tinker with the OS and access the bare-metal from programs. Writing self-modifying code that changed the stack/registers as it ran was sort of fun. You could do supposedly impossible things!
Stupid old fart that I am....
