In general, a computer that has access to more than one magnetic drive (physically different drives, not a partitioned one) can perform faster, since the heads are locked onto arms and the entire drive (regardless of size or number of platters) can only read/write one file at a time. This is the same reasoning for the common recommendation of system/program drives being different from data drives. System/program drives should be the fastest drive possible, and often need to read quickly, with less writing (why SSDs excel here).
The best advice is what Geoff mentioned. Actually copy/paste a bunch of data to the new drive and see what its actual performance is. You could also copy a few project folders (or even "save as" from SONAR) to it and actually test to see how it performs working on a project that is resident on it.