Thanks @Phil Rhodes
I think @Steve Woronko is correct, that simultaneous writing would never be possible / advisable. Too much risk of data corruption.
It's not a Mac or PC thing, just a hard drive / buffer constant that means you shouldn't be writing that much data to the drive from two sources at the same time.
On the Apple laptop point – I understood that PCs still can't properly write ProRes, which is what 99.9% of post-production houses want.
Do you know if they can?
Thanks