ISTR back in the early days of (build your own) PCs having an old 486 PC with two hard drives, and hacking the "Turbo" switch on the front panel (remember those?) to be a read-protect switch for drive D - i.e. when the switch was one way the hard drive became like a CDROM, and you flipped it the other way when you wanted to write to it.
OS on the r/w C drive, all your data on the write-protected D drive.
Granted in those days drives were IDE 40-way ribbon cables with a separate R/W line, but surely this techniqe is implementable in SATA?