@R.Belmont & O.Galibert: i made a partial disassembly of the Z80 part (located at the end of the Rainbow-100 B BIOS).
Sector reads & writes are handled there - as far as the BIOS is concerned.
Perhaps we find out what's wrong with the timing?https://dl.dropboxusercontent.com/u/37819653/BANNISTER/Z80_DISASSEMBLY__2016_02_04.html
(you may want to search for WRITE_SECTOR)
Thanks for having a look.