After working for a long time to finally get a MFM hard drive working in my IBM AT, I finally succeeded. I had no idea how to do this, having never worked with this type of hard drive before. All I had done was put a 1.08 gig IDE hard drive in my 386.

Here's what I did:
1. Got a 16-bit hard drive/floppy controller. Installed it in the computer.
2. Used the IBM AT (IBM XT-286 also) setup disk I downloaded from http://www.can.ibm.com/helpware/vintage.html (link broken, but left intact in case you want to try to find it yourself)
3. When I ran the setup program, I selected 1 hard drive, and selected type 2, since that was what was correct for the hard drive I was using, a Seagate ST-225.
4. I booted off the floppy disk, the last time from having no choice, hopefully.
5. I had downloaded a program called HDAT from http://www.computercraft.com It had a low-level format program included in it.
6. I ran the low-level formatting program.
7. I then used dos's FORMAT command to format the hard drive as a system disk.
8. Having dos 5 put on the hard drive's boot sector, all I needed to do was to install dos 5 the rest of the way.
9. The computer booted off the hard drive for the first time!!!

The steps I followed for my newer 286-16 motherboard were almost exactly the same, but I installed dos 6 instead of 5, and the BIOS supported more hard drive types, so I could use my Seagate ST251.