why no mention of Everspin?
they seem to have actual working MRAM, used by LSI and Dell. Unless it is no longer used or for some other reason "does not count" ?
The era of flash memory is anticipated to run out of road in the 2020s and newer technologies involving resistance and electron spin are poised to take over, delivering higher capacities, greater speed and DRAM-style addressability. Some people ask if one of these new technologies could actually unify dynamic memory (RAM) and …
They have working MRAM, but it is in megabit densities per chip. They are 'working on' gigabit densities per chip, but with other technologies moving to terabit densities per chip (already there if you count stacking) it isn't really competing in the general purpose storage market with flash and the other contenders mentioned.
A disc doesn't have to write a full block, it's the file systems that do that.
Not all file systems use blocks, and those that do generally allow you to choose the block size if you want a different tradeoff between storing the location of the data and the data itself.
Actually, you do have to write a full block in a disk drive, like it or not. Parity and error correction bits are spread throughout the sector in a disk drive, which means if you were to attempt to change less than a block you'd destroy the ECC, munge the SOVA, and corrupt the detectors. You could modify the file system interface to hide the fact that you're not writing a complete sector, but the drive itself has to write a complete sector.