URGENT: HD trouble

My previous HD started showing signs of breaking down and I thought no further: I went for a new one.
All seemed to went fine, until...

The new HD refused to boot OS4!!!! Well, sort of. It does boot up if I have an IDE cable plugged in the OLD drive. As soon as I remove said cable, U-Boot complains it can't find the SLB on any drive. I put it on the new drive, obviously (same slb I had on the old one).
I have the "master" jumper set on the new one and have it set to "bootable".
I've tried changing cables between all devices, but nothing positive happens. Both drives are same model and capacity.
I want to remove my old HD to have my CD-writer back, but I can't do it...
It seems that the old drive is still calling the shots, even without the power cable attached.

Can anyone help me out? I was never very fond of HD installations, but always managed to work it out in the end. Not the case now. I'm sarting to despair.

OS4(update4), with onboard IDE.

Have you changed the jumper on your old one to slave?

ace

Have you changed the connectors?

It should be Mobo -----> Slave -----> Master (80 wire UDMA cable)

There are some cables around that have the Slave connector on the end rather than in the middle, but the fact remains that Brandon needs to make sure that the new drive is connected to the Master connector (the black one).

It's on the master.

Regardless of that, shouldn't the U-Boot screen (at least) appear? I get just a black screen!

I thought I had it solved, but no. I managed to get booting, but it's as if the drive "loses" the slb file.

but, do u see it in mediatoolbox if you start the amigaos with a bootable cd?

Yes, the HD is always visible. It just doesn't seem able to boot when switching on the machine. After that and after using the CD to get to the UBoot screen, it works as usual...

so is not a jumper problem, but is strange that it doesn't boot at all..

Even stranger is that it just shows a black screen.

And if I do a total reboot (the one that brings you back to UBoot instead of AOS4), all's fine!
The 2 boot procedures behave as they should.