retrocomputing.stackexchange.com/a/30820 - Looked way too deep into WD XT hard drive controllers, just to tell someone their broken drive is broken... ;)

The architecture of the WD XT hard drive controllers is really interesting (that's why I added that in footnote 1) in that the WD1010 chip actually is a fully functional hard drive controller, and they added an 8048 microcontroller just to translate from the IBM/Xebec protocol to their native controller interface, which WD could conveniently get rid off when IBM moved to the AT, and made the native WD1010 interface the "new standard" supported by the AT BIOS.

The WD1010 interface is basically what evolved into ATA.