Did you build your own Fusion Drive? It seems like you must have opened your iMac as that config isn't available as a BTO. I would caution others against this route, as:
a) It is currently untested, and slightly unstable. At the Apple Store we had some serious grief maintaining even Apple supported implementations of Fusion Drive.
b) You have to cut the adhesive holding the display in place.
c) Totally voids any semblance of AppleCare you ever had.
Yes I did.
A. My drive is untested and unstable? or all fusion drives?
B. Uh? Nope.
C. My iMac totally voided any semblance of AppleCare I ever had all by itself when it reached the age of 12 months. They should teach you guys this stuff.
A. All Fusion Drives, at least when compared to a standard HDD or SSD setup. They have certainly gotten better, but we're eagerly awaiting 10.9.1 updates.
B&C -> I had assumed you were running a late 2012 or newer iMac. Which would have had AppleCare still and had the adhesives.
If you have a "thick" unibody iMac, then you don't have to worry about adhesive, but you do have to get the glass clean, which is probably worse. I hated working on unibody iMacs for that reason alone.