Caching HDD Files on an SSD
Other World Computing announced a product to do this at CES 2015: Transwarp.

From their press release:


  Utilizing any SSD as a removable, flexible cache, Transwarp magically
  brings the incredible performance of solid state drives to any
  high-capacity hard drive. Transwarp [...] is slated for release later
  this year.
  
  Key Features:
  
  
  Combines any SSD and HDD into an intelligent, fast volume
  Achieve the incredible speed benefits of SSDs with the capacity of traditional hard disks
  Quick and easy set-up
  Maintains all data on the HDD
  Compatible with any filesystem supported by OS X