Limited lifespan isn't the reason why one should store data files on a different drive. Yes, flash memory does have a limit to the number of write/erase cycles it can handle. But, even the worst case, that limit is at least 10,000 cycles. So, with a 250GB drive you would need to write and erase 2,500,000GB of data to reach that limit. If you keep your drive for 10 years you'd have to write 250,000GB a year, or 685GB every single day. That's erasing and refilling the drive two-and-a-bit times every day.