Personally, that's the approach I'd prefer, because then all my data is on the drive, which I can take out of the system when the power supply fails, to recover

Personally, that's the approach I'd prefer, because then all my data is on the drive, which I can take out of the system when the power supply fails, to recover my data. If the OS has silently migrated some of my data onto an invisible chunk of flash, I can't recover without that flash. (If the OS is just using the flash as a cache, OK, but why? It's slower and more expensive than RAM.)