I use a 2011 Macbook Pro to play live with two M-Audio keyboards. I use Ivory II and now I'm purchasing Omnisphere. I was told that it would better to put the libraries for these very large programs on an external drive, preferably SSD connected by Thunderbolt. Can someone explain how that is better than putting them on my internal spinning hard drive? I still have plenty of room on my internal hard drive since I only use this computer for this purpose.

SSD have faster read/write speeds than the SATA internal drive on the Macbook Pro's and Thunderbolt has a throughput of up to 10GB/s. It is also advisable to keep your audio data and projects on a separate drive to your system for better performance.