How can I write high-speed A/D data to an SD card?
You may not be able to do this with a PIC33, and almost certainly not just any PIC33. Some flash cards 'go away' for long periods of time (tens of milliseconds or more) despite the high throughput this latency can kill you with a small microcontroller because you'll run out of buffer RAM. 

If you allow buffering for 200ms (which should be safe) then you will need 20K words of memory, which a PIC33 may not have (the biggest members of the family have 48K x 8, so if your ADC takes two bytes that's 40K, and presumably your program needs some RAM. 

At 100,000 samples/second you also won't be able to do much processing on the data, just grab it. I'd suggest using RAM. And you'd have processing power to spare if you used an ARM with an external SDRAM or even serial RAM.