Normally, the filesystem wouldn't directly twiddle the disk hardware; even on the PDP-11, you had a bewildering array of disk types: fixed head RC disks, single

Normally, the filesystem wouldn't directly twiddle the disk hardware; even on the PDP-11, you had a bewildering array of disk types: fixed head RC disks, single platter RL01/RL02, multi-platter RK01-RK07, RP disks that were a somewhat more advanced RK (but completely different interface), Massbus disks like the RM04, MSCP disks which were a very weird beast (the programming interface was not entirely unlike io_uring), etc. (to say nothing of the 4 different tape drive interfaces, two different floppy drives, and a mind-numbing array of serial interfaces) v6 unix already had support for most of these, and even the famously tiny RT-11 (which would happily run with 8k of core for the kernel) supported multiple different types of disks at the same time.