The sound-on-sound tech editor has a point, but there are other things to consider as well.
1) Maximum throughput of the interface between the processor and the "disc" : with multiple discs, potentially using different types of interfaces, the maximum throughput can be a lot higher then a single disc.
( there's currently no single SSD capable of using the full bandwith of thunderbolt / PCI or similar, let alone the full bandwith provided by the I/O chips on the motherboard)
2) SSD's that are re-written a lot, add internal copy/write overhead ( the so called write multiplication factor)
So basically, if you want to keep your library-discs as fast as possible, don't use them to write things on on a regular base ...
Also : don't put the VSL time stretch Cache on your library discs !
What is important is having a fast disc for (system) caches and stuff like that (R/W), for audio (R/W) , and for sample libraries (Read only).
I don't mind waiting a bit for an application or the OS to start, so if it would be just for that, I wouldn't need an SSD (that's a personal thing though, some people want a PC to start up as fast as possible).
However, for pragmatic reasons OS / applications / caches normally reside on the same disc, so if you can afford it, you'll still want this disc to be an SSD too.
Audio : If on the same disc as the OS/app/caches an SSD is must. If not : depends on number of tracks / sample rate.
Sample Libraries : definitely SSD ! Don't put it on the same disc as the audio : the two BIG data streams (audio / samples) will go to the same disc , on top of that the "write multiplication factor" start playing a role if you write a lot of audio
Conclusion
If you can afford a fast external "disc" , do so.
Prefarably Thunderbolt (fastest AND more stable - less hickups - then USB 3.0) or SAS ($$), or USB 3.0 / dual eSATA.
You can choose to put audio or samples on the external disc, depends on your particular workflow / mobility demands.
- If you have really big / multiple sample libraries, it makes sense to take an enclosure that supports multiple SSD's in a RAID configuration.
(I assume we want to stick with the "reasonably priced" stuff, muldi-disc multi(parallel)interface enclosure are out of scope)
- If you want to be able to do audio work on the road without an extra additional enclosure, it makes sense to put the audio disc internally
- If you want to "share" (physically de/reconnect) the project/audio disc with others / or with other systems , it makes sense to put the audio disc in the external enclosure.
Looking forward to hear about your future "acquisitions" !
PS : I don't know what kind of MBpro you have, but make sure the throughput in the optical bay slot is high enough ( I believe the current models have equal characteristics for main bay / optical bay).