Playing Monday Morning Techie-Quaterback here (Don't Shoot! 🙂 ):
I'm begining to wonder if this isn't a problem with the new (to the MacBook anyway) Intel chipset? Losing access to the internal SATA would show up as a freeze with some window updates as has been described, losing access to the graphics frame buffer would explain the screen freeze (except hardware mouse pointer). Postulated failure modes:
1)The new "lite" GPU is in the chipset and it's being switched, a software or hardware/timing error occurs and for whatever reason the driver is unable to reset access to the boot drive and thus either of the freeze types.
2)For whatever QA reason (bad batch, connection, drive failure, intermittent whatever) the chipset glitches, possibly aggravated or increased in probability by disk load, the driver is unable to reset access to the boot drive and thus either of the freeze types.
On the hardware side: it sounds as if most of the hard, repeatable or frequent freeze machines were all custom or at least mail order? Can we confirm this?
On the software side the OS X driver for the Intel chipset may need some work and a patch, especially with regards to error handling.
Comments on the above?
James