Here's what I did to install in a Tandy 3000HL computer the following:
  A Seagate ST08 16-bit IDE AT HD/FD controller board
  A Seagate ST1144A 125MB 3.5" IDE HD 20ms hard disk
  Tandy 1.44MB 3.5" floppy disk drive (FDD)
in addition to the 5" floppy which came with the 3000.
I was working with no doc on the drives or controller board,
so I thought this might save someone my trial & error tribulations.

BASIC HARDWARE, 3.5" FLOPPY

I first tried to fit a Toshiba 3.5" floppy in, but there were
hardware-fit problems that caused me to just give up.
The Tandy floppy required a 3.5"-5.25" adapter kit.

BASIC HARDWARE, HARD DRIVE

(I will leave the joyous nightmare of fitting the 3.5" hard drive
into the 5.25" bay using the 5/8" rubber spacers for you to
discover for yourself. The Gods of technology be with you.)

CONNECTING CABLES

Two cables run from the controller board to the drives.
One connects the HD pins to the hard drive,
the other connects the FD pins to the 3.5" and 5.25".
These should be fairly obvious, although I did manage
to get the FD connected backward to the controller board at first.
You'll know right away if this happens. The drive lights all
come on and stay on after you boot up.

JUMPERS

The E6/E7 jumper is removed from the 3000's motherboard.

On the controller board, there is a 2-pin jumper at JP4 or J3...
the labelling isn't real clear--it's the 2-pin not the six-pin jumper.
This had a jumper on it when I got the board and is required.

Also on the controller board there is a 4-pin jumper at J3, near
the FD bus. The two pins =farthest= from the FD bus must be jumped.

BIOS & DOS

I don't know if it was *really* required, but in the process of getting
this all set up, I also replaced the BIOS with Tandy's version 1.03.02.
I also upgraded to MS-DOS 5.0 from Tandy.
With BIOS 1.03.02 you need at least MS-DOS v.3.20.02.

PRAYERWARE

The above installation failed to work until I whispered a plea to the
spirit of technology. Couldn't hurt.