Caching HDD Files on an SSD

Question

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.

Answer 1

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

Answer 2

While third party solutions might be or become available, you might want to use those with caution. Like many low-level integrations, there is a risk that any update from Apple might break the functionality (in a graceful or maybe in a really bad way, i.e. data loss) of that third party product if the developer cannot keep up with the updates of the operating system.

Other than OWC's solution as presented earlier, you might be able to simply make an automator action that creates a fusion drive on the fly when you connect your drive, and stops it when you unmount it. While you won't have a persistent cache, this might work just fine for you depending on your usage pattern.