Well, start saving for that SSD RAID setup, then.

I am not sure Fraps does capture true 4k. I have the latest paid edition of Fraps and if I take a screenshot it shows ragged edges that are not in the original image.

I think your math is off by a few orders of magnitude....

3840*2160*24 = 199065600 bits per frame

199065600 / 8 = 24883200 bytes per frame (~25MB)

24883200*30 = 746496000 bytes per second

746496000 bytes = 746.5MB

I was referring to your claim that a 2 TB HD would filled in 3 seconds at 700-750mb per second.

Ah yeah, you got me there. Sorry!

Get the complicated bit right and stuff up the simple part. Typical.

Looking at these two articles (pretty opaque)
it appears they must just compress like crazy.
How else do you get a 7680x4320 pixel image
over a 700MHz carrier frequency @ 60fps?

https://en.wikipedia.org/wiki/Ultra-high-definition_television
https://en.wikipedia.org/wiki/High_Efficiency_Video_Coding

It says that that higher res (I'm not seeing the figures
for 3840x2160 immediately) has 48Gb/s uncompressed,
50Mb/s compressed, so 4K must be around 12Mb/s compressed (?!)

It also says they store 16 frames to predict upcoming
frames as part of the decompression, so I guess they are
doing all those cheats they can pull off, like where
possible using the data from the previous frame where
that hasn't changed, or is only shifted as the camera
pans.

...16 frames buffer - consider the associated latencies in games. Maybe not so much an issue with flight sims, definitely one when it comes to action-oriented titles.