Caching HDD Files on an SSD Given a system with a fast internal SSD, and a slower external HDD, is it possible to improve the HDD's performance using free space on the SSD without creating a Fusion Drive? In my case the external drive is always connected (the system needs the extra capacity).

Really I'm looking mostly to use a big chunk of space for read-caching, and maybe to buffer writes (provided the content doesn't stay there for long, and is safely written out before being unmounted).

Unfortunately the SSD isn't big enough to take the full contents of the HDD, and I'd like to avoid having to manually symlink folders, though I may do it for some temporary/cache folders where it won't matter if they're moved.
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