Take this with a grain of salt because I'm far from an expert.
Correct me if I'm wrong, but SSD's are usually used for the server's Operating System because they have much faster read/write speeds than normal HDD's. One of the downsides to current SSD's is that they have a higher failure rate and degrade slowly with use. The physical, magnetic media used by HDD's is much more reliable, and cheaper to buy with massive storage capacities. I would also suspect that the HDD holds much of the "meat" of the data (player data, tables and such, etc) but again I could be totally wrong.