Tested five cf hard drives that work in my 1200 and neither of them would boot only if i used my fast ata card also i m based in the uk
What is the Amiga 4000d specification ? What is the hard drive that you are using ? What processor do you have and what memory ? etc etc. All of this could have a direct impact on whether your machine has a problem with the actual hard drive. What do you see on the screen when you boot ? Are you detecting a drive with the two mouse button down? Are you actually talking about a honest to goodness physical 3.5" hard drive IDE or just a flash drive ? Can you boot on a floppy ?
For me the last thing I would jump to any conclusion would be the hard drive. I would be checking that my memory simms were seated properly. Specially with the heat we have been getting in the UK of late. The first one in the rows would be my first port of call.
Not sure how you would fit a 1200 Power Flyer to a 4000. I assume you have the 4000 version.
Have fun.