The 50-021 / -031 lacks the ESDI part of the BIOS.

Peter Wendt scrawled:
   If you pick the 50Z BIOS chips (2 x 512) and split-burn them down into 4 x 256 (as the 50-0xx uses them) you can convert a 50-0xx into an "almost 50Z" without needing the "50-to-50Z upgrade controller", which is a rare item. It only adds the ESDI code to the BIOS and needs a different @DF9F.ADF, which allowes to set an adress for the BIOS ROM on the controller.
   Did that a decade ago and it seemed to work fine with a 50-021 and a 120MB HD from a Mod. 70.

>Couldn't you also replace bits of the BIOS in software?  Get the BIOS copied into RAM as is sometimes done for performance, then overwrite bits of it.  I dunno if anyone has actually done this, but it sounds plausible.

   The 286s don't copy the ROM into RAM after POST like the 386s do - and even if: you should do that *before* POST to have the ESDI controller available for the boot process .... but I don't think that was what you *meant*.
   [Rem: The "upgrade controller" in fact *does* something similar: it copies some routines into an appropriate area in the "adapter ROM area" (C000 - DFFF) and sets the bootloader entry point to the start of that routine. Usual "check adapter segments for bootable controller ROMs" stuff.]
   Using "Split Copy" on an Eprommer is much better. You pick the "even" of the two 27C512 (64K x 8 bit) Eproms, read and store 0000-7FFF in the first and 8000-FFFF in the second process. 
   Then you pick the "odd" Eprom and repeat the step. The four files nicely burn into 4 x 27C256 (32K x 8 bit) Eproms and you have Odd-Low, Odd-High, Even-Low, Even-High.
   It's *a little* complicated - but not much. (Even I managed ...)  ;-) I think I still have the codes in my data cemetry somewhere.  In the 50-021-format.

The advanced Do-It-Yourselfers *may* pass this obstacle with this method:

- copy the 50Z-BIOS from 2 x 27C512 into 4 x 27C256 (split the "ODD" and "EVEN" eprom into ODD-Low and ODD-high, EVEN-low and EVEN-high. 0000h - 3FFFh = low, 4000h - 7FFFh = high at each set) This requires an Eprommer !
- install the new BIOS chipset on the 50-021 planar
- use the bare "Hardfile Riser card" 90X9441 from the 50Z
- use any of the IBM ESDI drives (30-160MB)
- reconfigure the machine. "1 ESDI harddisks" must be displayed in the
device-list on advanced diagnostic (unmodified 50-021 show 0 ESDI drives here on that attempt ...)

The 50-021 uses a proprietary MFM / ST-506 controller along with a CMOS version of a MFM 20MB harddisk - and that's the only drive supported on that controller.