We're talkin' a terabyte per hour. So, if you're running RAID5, that's 2 terabytes per hour.
Technically, RAID1 would be 2TB/hr. RAID5 requires 3+ drives and the amount of space you "lose" is relative to the number of drives. 3 drives loses around 33%, while 6 drives only loses around 18%.
Regardless, the file sizes of 4K RAW is quite unwieldy. I anticipate only using it for specialty projects that don't require a ton of footage (a well-planned music video that doesn't require 20 takes of full-song performance coverage). Then I would probably transcode the footage to ProRes for editing, then maybe drop $100 on an external to keep the RAW files around. Not the most elegant/reliable solution, but I think if I'm able to create high-quality 4K ProRes files from the RAW, that's good enough. Retaining the RAW files is just gravy after that. I certainly love that CD was able to get Sony to allow capturing the RAW, but the reality of the situation is that high bit-rate/depth compressed files are the best for workflow.