A low-level format is defined as writing sector marks and everything else needed to a blank magnetic media (or treat it as such).
On MFM/RLL (ST-506) drives this was needed since every controller/system used a unique timing, so it wasn't possible to do in the factory. Later, ESDI drives somewhat softened the distinction between LLF and re-zero (on some models is was a real LLF, on some a re-zero and prep for partitioning).
Yet later, SCSI and IDE drives were usually based on linear actuators, rather than the earlier steppers and had to rely on servo informations (either a dedicated platter or embedded info between data sectors) that had to be written in the factory to enable the heads to stay on a track. For physical reasons the drives are not able to write these servo information all by themselves and thus cannot be LLF. Trying to do so on early IDE drives may wipe out bad sector tables, vital servo information and/or the drive's firmware (only a small bootstrap resides in ROM/Flash).
Beginning from the early 90's, drives all run on linear actuators (much faster, cheaper and more reliable) with embedded servo info and can only mimmick LLF.
