I was thinking you wanted to recover some data from the MFM drive - if the drive is prepped and autoboot is activated, it should be able to boot up by itself. Prepping it again will erase all data on the disc.
If autobooting doesn't work, you can copy the HHDisk driver from the A2090 disk to your Workbench disk's Expansion drawer.
You'll have to edit devs/mountlist as well: copy the RES0: entry from the A2090 disk to your Workbench:devs/mountlist - hopefully the parameters fit.
After rebooting (or 'binddrivers' in a shell console) the HDD should get mounted. I can't remember the exact way to do it any more and the manual is a bit vague.
BTW: Autoboot is disabled by closing J4, so you might only have to remove that jumper.
BTW2: the A2090a has got three ROMs:
U50 HI Top centre, below J2
28 pin 315098-01 2090 AB5C
U51 LO Top centre, below U50
28 pin 315097-01 2090 D32B
U36 Top right
28 pin 390230-09 2090a 2FEF
U50&51 are the boot ROMs, U36 the Z80 firmware
