SPCR

LaCie d2 Thunderbolt 2 with SSD booting Mac as Fusion Drive would be awesome!

LaCie d2 Thunderbolt 2
https://www.lacie.com/us/products/product.htm?id=10628
LaCie d2 SSD Upgrade - Convert LaCie d2 Thunderbolt 2 to External Hybrid Storage
https://www.lacie.com/us/products/product.htm?id=10631

Indeed, it is possible:

REVIEW: LaCie d2 Thunderbolt 2 with SSD upgrade configured as a Fusion drive
http://www.barefeats.com/hard191.html

But how? LaCie and Apple should collaborate to make that officially certified.

Noisy and expensive. No thanks.

Certainly the device with the little fan would be audible -- as would most 6TB drives placed on your desktop. But I'm not sure just what the benefit of a single large boot fusion drive is. iirc, there's been reports of less than ideal performance w/ the fusion drive. I'd personally rather have a PCIe SSD for boot/OS -- tho admittedly, that won't work for a laptop -- and if doing something like large file HD video editing, go entirely SSD, PCIe or SATA3.

Vicotnik wrote:Noisy and expensive. No thanks.

+1
A shame that thunderbolt is still mostly a Mac thing.