I am the original author of, among other software, CBHD, a widely known hard-disk driver for Atari ST and TT computers. CBHD was part of my book Scheibenkleister. After the book went out of print, I gave CBHD a public-domain status. CBHD is now maintained by Steffen Engel. Current versions can be found on the following servers:
BTW, Steffen Engel also wrote an excellent article on SCSI (German) which explains some of the background required for disk driver programming.