Apple have published a
article on the Fusion Drive. Lots of Q&A's
"Presented as a single volume on your Mac, Fusion Drive automatically and dynamically moves frequently used files to Flash storage for quicker access, while infrequently used items move to the hard disk. As a result you'll enjoy shorter startup times, and as the system learns how you work you'll see faster application launches and quicker file access. Fusion Drive manages all this automatically in the background."
Yes, that sounds great for small files like word processing or spreadsheets or Photoshop documents. But what about huge HD and 4K video files? They take a while to move. Plus the Event and Project folder structure. My guess is all FCP events and projects would be stored permanently on the HDD. If not, they'd eat up space on that tiny SDD pretty fast. Not to mention if you come back to work after lunch, and the Hybrid Drive is still moving the large video files you're about to work on?
It does say that you can add one partition to the HDD portion of a fusion drive, but the extra partition will not be part of the fusion volume... at least that's how I'm reading it. If (and it is a big if) the fusion drive is smart enough to keep the OS and frequently used apps on the SDD then it may be OK to put your video files on that second partition as the drive will be otherwise infrequently accessed when video editing.
I will be ordering a new iMac next month. Not sure yet if I'll order it with a fusion drive.
Since you can't put a SSD in on your own anymore, you kind of have to get the fusion drive. I put a SSD in my iMac and I'll never be able to use a regular drive again. They should have called the fusion drive lightning, anytime I use a computer that boots from a HDD I feel like I'm using dialup Internet from the '90s.
You're gonna edit with an external thunderbolt drive anyway so it's really a pretty easy choice. Also, since it's a version 1 product you have to get AppleCare.
Just get the 1TB fusion and use all that for normal computing. Thunderbolt for all FCPX.